hes twelve on the beginning but in season two in "mystery train" its his birthday and he turns 13. thats why in the season finale of season 2 when PB turns 13 he says "13..thats how old i am."
He was 12 but then there was a B-day episode with a train and he turned 13 so hes 13 right now. He will have another b-day this Jahr in like sept. oder somewhere near there