Lorenzo Cain is the best all-around free-agent outfielder available this winter, but for all of his obvious skills, interested teams have to wonder about two important questions before they agree to sign him to a long-term contract.
First, he'll be 32 years old in April, meaning a new club may be signing up for his mid-to-late thirties. Second, as a player best known for elite speed and defense, there's a few recent examples of defensively talented outfielders signing huge deals, and then providing little return on the investment. Jason Heyward's struggles are well-known, but detractors might also point to Carl Crawford, Michael Bourn, Jacoby Ellsbury and Melvin Upton Jr. as some regrettable signings.
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Fair questions, to be sure, but those aren't necessarily the same kind of players as Cain. Can we look back through recent history to find better comparables, and see how those players ended up performing? Sure, we can.
First, it's important to remember that while defense may be Cain's calling card, it's not his only skill. Last year, he hit a strong .300/.363/.440, which gave him a 115 wRC+. In other words, he was 15 points better than league average, the same as Mike Moustakas and Christian Yelich. Over the last three seasons, his line is a similar .299/.356/.445; that's 16 points better than average (per wRC+), and about the same as Marcell Ozuna or Francisco Lindor.
He can hit, is the point, as well as field (he finished fifth in Outs Above Average, the Statcast™ range-based outfield metric, with +15) and run (he was in the top 4 percent of speed, according to Statcast™'s Sprint Speed, similar to Trea Turner, which gives him plenty of room to decline). So in order to look back, we need to find players with similar skills at a similar age. 

The way we've done this is to look for outfielders who fit each of these criteria in a three-year span comprising either ages 29-31 or 28-30, with a minimum of 1,000 plate appearances in that span, dating back a quarter-century to the first wave of 1990s expansion in 1993.
• Offense between 10 and 20 points above average in a three-year span (per wRC+)
• Defense worth a positive WAR value
• At least 25 percent of playing time coming as a center fielder
This gives us a more appropriate pool of players to look at, unlike Crawford, who had his last good year at 28, and Heyward, who is only entering his age-28 season in 2018. While the 25 percent in center field requirement costs us corner outfielders who would have been capable of playing center, like Ichiro Suzuki and Alex Gordon (plus also interesting names Geoff Jenkins and David DeJesus), ultimately we're left with 10 names to look at:
Garret Anderson
Mike Cameron
Brett Gardner
Curtis Granderson
Torii Hunter
Andruw Jones
Kenny Lofton
Reggie Sanders
Shane Victorino
Randy Winn
It's an interesting group of names, if not a large one, and to that point, it does help illustrate that he's something of a unique player. Plus, we'll have to exclude Gardner, because he's only just completed his age-33 season, so we don't yet know how he'll age.
With our nine remaining names, how did they perform? And for how long? Eight of the nine outfielders played through at least their age-35 season, with Victorino (who last played at 34) the exception. Anderson, Sanders, Hunter, Lofton and Cameron all played through age 38 or older, and Granderson, who will be 37 in March, is a free agent who may yet land another job. Hunter, in particular, is a nice comparison, as he had a nearly identical stat line to Cain through age 31, at which point he signed a five-year, $90 million deal with the Angels.

If we take them as a group (not including Cain), we can look at how this all ended up going.
How did they do from age 28 to 30?
Averages: .276/.345/.470, 22 HR, 142 games, 611 PA,113 wRC+, 4 WAR
How did they do from age 29 to 31?
Averages: .273/.343/.466, 22 HR, 143 games, 608 PA, 113 wRC+, 3.8 WAR
(For reference, a two-WAR season is considered "league average," and a four-WAR season is star-level. Every single one of our players was named an All-Star at least once.)
So that's what they did up to Cain's age. What about after?

At age 32
Averages: .271/.339/.437, 16 HR, 108 games, 456 PA, 103 wRC+, 2.2 WAR
This first year was actually hit-or-miss for our group, although some fluke injuries were to blame. Cameron played just 76 games before seriously hurting himself colliding with Carlos Beltran in the Mets outfield, while Granderson played only 61 games around a pair of broken bones suffered when he was hit by pitches. Winn and Sanders both struggled in the midst of otherwise long runs of success. 
All told, this wasn't the group's best year, combining for a 103 wRC+ and 2.2 WAR, making them roughly average, though there were success stories -- Victorino had the best year of his career in his Boston debut, while Lofton and Hunter each had strong seasons. On defense, however, Jones, Sanders, Anderson, Winn, Granderson and Victorino never played center again regularly after 32.
At age 33
Averages: .271/.340/.455, 18 HR, 124 games, 516 PA, 109 wRC+, 2.1 WAR
At the plate, this was a much better year. Six of the eight players stayed healthy enough to play every day, and six (not the same six) were average or better hitters, including Jones, who had a late-career rebound with the White Sox and Yankees. This was the beginning of the end for Victorino, who played just 30 games due to injury and retired after an injury-shortened age-34 season.
The group improved to a 109 wRC+ line, again led by Hunter, who had a strong .299/.366/.508 season for the Angels. Overall, their WAR held steady at 2.1, as Anderson and Granderson took a step back with the glove.
At age 34
Averages: .266/.338/.428, 16 HR, 131 games, 504 PA, 106 wRC+, 2.4 WAR
Even with neither Victorino nor Jones reaching 250 plate appearances, the group still stayed healthy enough to average 131 games played. And while it's easy to forget how effective late-career Jones was, he did hit .247/.356/.495 in part-time play for the Yankees. Hunter and Granderson each had another strong season at the plate (131 wRC+ for Granderson, 126 wRC+ for Hunter), while Winn had the strongest fielding year of his career, although he did it in right field.

At age 35
Averages: .259/.331/.451, 17 HR, 116 games, 468 PA, 95 wRC+, 2.0 WAR
Victorino was finished at this point, but we have to account for that being a possibility for Cain, too, so we can't ignore it. Adding in his zeroes drops the group's average plate appearances from 527 to 468, and from 107 wRC+ to 95 wRC+.
For those who played, this was generally a strong year. Cameron combined 25 homers, 17 steals and strong defense to post a 4-WAR season playing in center for Milwaukee, and Sanders had one of his better seasons, hitting 31 homers with a .285/.345/.567 line for Pittsburgh. Hunter, Lofton, Anderson and Granderson all had above-average hitting years as well, though Winn and Jones lagged behind.
Beyond
Jones and Victorino were done by now, and Winn put up a weak age-36 season before retiring. But Sanders had 56 more homers in him and a .263/.325/.483 line after 35; Lofton, playing until 40, stole 114 more bases and hit .301/.363/.419. Granderson and Cameron remained average hitters after 35, while Hunter hit 72 more homers and remained an above-average offensive player through his age-38 season.
Of the group, only Lofton and Cameron remained regular center fielders, but you can see a similar path for Cain, too. He's already played right field in the past when Jarrod Dyson was in center, and it's easy to see him there regularly after two or three more years in center.
You shouldn't expect Cain to have five star-level seasons in the next five years, as he ages into his mid-thirties, but the point is to show that the outcomes can be good. Sure, he could be like Victorino, with only one more good year in him. He could also be like Hunter, or Cameron, or Sanders. We've seen this work out before.