When it comes to drawing people of different ages, there's several things you can do to indicate age. Mostly basic things like: proportion, costume, attitude. When I drew this, I didn't really study real models or anything, (which I should've but am too lazy) I just drew on what I generally knew.
Babies and children look more the way they do with a big forehead and their eyes, nose, mouth proportionally lower on their face. Their head to body ratio favors a big head and small body. Babies are fat and their leg an arm curves are often drawn like this ( ) curve against curve. They wear diapers or other cute suits that only a mother would forcefully impose.
Growing to children, the proportions grow. The head to body ratio starts to balance itself out. Here I drew a boy and a girl at odds with one another. To them, inter gender relationships are still yucky. This pairing also suggests some mischievousness on the boy's part. Girls also reach puberty faster than boys so at a certain age, they're taller, beefier than boy peers.
Continuing on to teens, the same proportional trend occurs, ironically the head gets smaller and the body gets bigger but they don't become dumber. Here, I drew this young man in a letterman's jacket, something only a high schooler would wear. His hair's all messy. And the girl's carrying some textbooks. I don't know if real kids would do this but he's being very gentlemanly towards the girl, holding open an imaginary door or whatever. Maybe in the 1950's teens may have had such manners. These days, maybe he's trying to rack up score points. Who knows?
Adults, I placed in a large age range 21-64 but the ones I drew are probably like in their early 40's or something. Their bodies get proportionally longer and beefier. Bodies are made up of these ( l curves against straight lines. Maybe it's a married couple with kids. The guy's wearing those trendy nerd glasses and some dorky nautica coat. And has his "aren't I mr. artsy suave?" (or so he thinks) goatee. The lady has her hair long and strong like she stepped out of an overpriced salon or used overrated pantene pro V. Got more bling, the phat purse, a "I'm past 25 but I'm still sexy" (or so she thinks) cleavage dress.
Senior citizens just degrade into decrepitude. They get too fat or too skinny, wrinkles, age spots all over, hunching, sagging, drooling, limping, shaking, wearing old mothball sweaters, and those big visor things. I had to show them arm in arm together otherwise it'd be too cruel. They've lived a long life; hope it was worth it. What's left to do but go to the park and become tai chi masters?