I make most baby quilts about 45 x 60". If you buy a baby quilt panel, it's 36x45", and I always add borders around it, usually 5-6" all the way around, sometimes 1 border, sometimes several borders adding up to 5-6".
I figure that the kid can carry the quilt around with him/her for a lot longer if you make it a little bigger.
For kids 10-15, I'll make a double bed sized quilt. I figure it will fit on a twin bed, hanging to close to the floor. Or depending on who it is, I might make a lap quilt, or maybe just a fleece blanket 60 x 72"