I had my first two kids when I was pretty young, and I was already a bit chubby, but I worked out and managed to get a very flat tum, I still had a little tiny bit that was soft and the skin a bit more loose, but no one else would have noticed.
I was 29 when I had my third, and again my tummy didn't sag or wrinkle really badly, and I didn't get anymore stretch marks, I haven't got anymore this time either.
I think very few women go back to having the same flat tum they had before, but I would guess a great many of us don't feel that are tums are too bad, and you see plenty of women who shrink back very quickly and their tums don't show much wear and tear.
I guess everyone is different, either way, try not to worry about it, your tum will reflect your amazing hard work of carrying your child.