I was 36 y.o., my son was 9 mths old. I don't think I have had looked too oldish whilst I carried my son. Plenty of people didn't believes my exacted age due to I always keep my mind young and dressed up more fashionable particularly during my pregnancy.
I of course had came across some pregnant ladies who looks too old and horrible, but none of my business, at least they are happy.
For me, I reluctant to carry baby in such young(20's~30's), no financial supports & no regular place to live, headache ... Huh ...