Ok, oh idk if he is revealed at age, but i guess i can tell ya my opinion what i guess he is... IF it is episodes in the past where the others act like kids and such, Lumpy is maybe 20 years old even if ya shouldn't be a teacher at that age or idk… But when the characters act like adults then Lumpy is probably 30 years old, could be 39 but his current age is at least 30 imo so your welcome but since this is my opinion, i am not right but this is just my guess i suppose.
Do Tree Friends even have an age at this point? I mean, Cub's been a toddler since 1999.