Stamkos did not have his 2nd best season at 31 years old... It was his 5th best season. And if you account for projections in a lockout shortened season and an injury shortened season, it was actually his 7th best season.
Player profile page for players in the NHL
evolving-hockey.com
If anything, Stamkos is a perfect example of literally what I am saying. 5 of his 7 best seasons happened before he turned 23 years old. 6 of his 7 before 28.
Since his 28 year old season, he's had 2 of his worst seasons of his career. Last year was a strong year and now so far this year he's tracking to have another well below average year.
Just like I said. Exceptions to the rules can happen. A player can buck the trends by a couple years, a player can have an "out of nowhere" good season despite being over 30. Those aren't the "normal" and teams are very well aware of the aging curve.
Funny enough Bo Horvat follows the trend as well:
Player profile page for players in the NHL
evolving-hockey.com
Horvats best season came when he was 22 years old and he has pretty much stagnated every year since then. He's on pace to have his best season of his career this year... But you're running into the dangers of both a very small sample size(Not even 20% of a season yet) and the very real and well documented contract year boost.
I wouldn't be surprised at all if Bo has the best year of his career this year, but teams will be extremely cautious about the fact its coming in a contract year and coming after 4 years of stagnated play.
EDIT: Those links are probably behind a paywall. So take my word for it, or dont.
....In the grand scheme of life, sure 27 is obviously quite young. In the NHL, 27 is not young.