Plays gain experience from the minutes they play. Whether that be scrimmages, league games, national team games, anything but Private League.
While experience isn't generally very well known by the community, there are some agreed-upon effects of experience. The positives of high experience are that the players will perform better at last 5 minutes of the game and generally make better decisions throughout the game (make a pass to a more open shot rather than shooting himself). The effect is really seen in the end of games though.
There are not that many negatives to having high exp besides one of your better players might get subbed out for players with higher exp at the end of the game if the game engine deems them better. Not sure about that one though.
The only way a 26-year-old can hit legendary is if he was a tutorial player whose stats are semi-randomly generated.
Generally, though, the experience isn't that valuable of a skill, but those are the effects.
Hope this helps!
2x NBBA Champion: S55, S56. 3x USA Cup Champion: S54, S55, S56. WR for longest home streak ever at 11 in Season 47.