Should i gain weight in my late 20s?

i was never able to gain weight when i needed to for sports but now that im in my late 20s i cant stop puting it on. i have gained nearly 40 lbs in just over a year. i stopped smoking pot 18 months ago. and i stopped smoking cigarettes a month ago. out of the 40 lbs ive gained 12 of them in the last month. i am as active as i have been for the last 5 years. am i just getting to the point that i am getting old and have to start eating like my parents?