If you win big at
pokerstars, they usually hold your money for one to two weeks, just as a safety precaution.
I dont know about the age verification, but I know that there are some players who have been found beeing underage.
I know that the Australian player, James Obst, a.k.a. Andy McLeod had won several 100k on pokerstars, and then they found out he was only 17 years old.
They locked his account with all the money in it, and then it got unlocked on his 18th birthday.