Mark Jackson is still playing pro basketball on his 38th birthday, but the reason, he figures, has a lot more to do with his 21st birthday.
   
Before Tuesday's game, Jackson accepted birthday wishes from several Clippers fans in the Staples Center crowd, Los Angeleans who remember the two seasons he spent here a decade, and he challenged a few of them to guess his age. One said 27; another guessed 30. Nobody came close to the truth, which clearly delighted Jackson, one of the 10 oldest players in the game.
   
Especially because he doesn't feel 38, either.
   
"I feel fresh. I feel real good," Jackson said. "When I wake up in the morning, or when I run up and down the court, I feel as good as ever."