I think having children young (20-25) is probably better.
-You are young and strong enough to bear the strain
-You have enough stamina to do more things than one, i.e. study or work + children
-You will still be fairly young once your kids are gone from home (life doesn't stop at 40!) so you can have fun afterwards as well
-Your children will hopefully have you a long time before you die
-You might even get to see some grandchildren
I would not recommend having children just for the sake of it though. A stable relationship, at least one person earning money and a real wish for children should be there.
I often find that older mothers(>35) especially tend to be over protective.