I think about this all the time PZ!
I am 29 and DH is 36 and we don't even have 1 yet We wan't 2 children so if I'm lucky and conceive n the next 6 months, we will probably start trying within 12 months. Given my age and the fact it's already taken us over 2 years. I always wanted about a 2 year age gap, but I realise now that that may not be a good idea.