In Big Bang Theory Series :
In real life:
John Mark Galecki, known as Johnny Galecki, is an American actor and producer, born on April 30, 1975 in Belgium.

He is best known for his roles as David Healy in the sitcom Roseanne, Rusty Griswold in National Lampoon's Christmas Vacation and Leonard Hofstadter in the sitcom The Big Bang Theory.
Age : old.
Attribution - Sources - licenses