What age do when generally start gaining the most weight?

i have been trying to gain at least 20lbs for the past 4years. i am 5ft and weight 110lbs this is the highest i have been able to reach in weight even after having a child. I look healthy for my size people tell me, but of course, I am a women and will never see what they see... I thought my child could teach me something about gaining weight lol. Ive seen a lot of parents gain weight from eating what and when their children eat. But that didnt help me... So i tried shakes and pills but they werent good on my stomach. I started eating three full meals and three snacks. Exercising-cardio, I walk with my son everyday i am also on my feet all day- everyday where i work. Im guessing it just isnt my time. When will I?