If I were you I think I'd look down at my chest, then look around behind me; and say, "Hmm. I'm not seeing any sign around my neck that says, 'Other People's Opinions Welcome'". :)
REALLY! If you want another baby it's your business. A 20-year-old is pretty much grown, and 60 really isn't all that old. Yes, there's a chance someone could get sick and die at 60, but that's not how it is for most people. There's also a chance someone could get sick and die at 25. A lot of people find that having a child older helps keep them young longer. (Really - where is your mother-in-law from? The 1800's, or something, when people had five kids by the time they were 24?)
Chances are if you were expecting a baby your doctor would monitor you more closely than if you were 25, and chances are there would be recommendation for testing for any genetic abnormalities. People have babies at 41 all the time, though.
If your husband isn't sure that's another matter. If it's health issues associated with any risk to you or a baby, that's something that maybe you and your husband could talk to your doctor about - then decide. If he's not sure for the same reasons his mother isn't sure - well, that's a whole other thing; but, really, if he's still letting his mother's opinions color his own, that's a problem completely separate from the baby issue.
Having said all that, a lot of women start thinking they'd like that "one more baby" before they get absolutely too old to have another one. Some go ahead and have the baby. Others aren't sure, or their husbands don't want one; and they don't have one. It's very common to get through that time of thinking about that "one last baby" but move past it and get to where one is happy enough not to have gone ahead and had the baby.
My advice is talk more to husband about it. Talk to your doctor about it too. Don't talk to your mother-in-law about it at all. Surprise her if/when you do have a baby. :)