Hulk in MCU is 49 Years old. Fun fact: Bruce Banner was born in 1969. December 18, 1968 to be precise.
BTW Mark Raffealo, the actor who plays Bruce Banner MCU is 51 as on 2019.

Hulk in the MCU is 47-49 years old as per the movies invincible hulk to the endgame and in comics he was born in 1969

Banner is 49 years old. Hulk is 13 years old because the gamma accident happened in 2005 so by 2018 Hulk is 13.