Well, if a woman has a baby at the age of 20-22 (which is quite rare, but not unheard of), she can still be in her 30s when her child is 18. And, of course, there are many women aging between 38-44 who look outright gorgeous. But, yes, in most cases, you would expect the mom of a grown man to age at least 45-50. And unless you are Tom Cruise, it is impossible the hide the effects of old age at that point. I'd be okay with an younger looking mom, although I would expect the game to explain why she looks so young (assuming, of course, the game is trying to portray a realistic story).
However, sometimes there's an even more serious problem with how the moms behave. At older ages, specially when you become a parent, you tend to become more mature and pragmatic, it's hard to explain, but most of the the time, older people tend to show different behavioral traits than younger people. Some games tend to overlook that and portray the mom as some naive and immature character, or the act mature only superficially. That seems totally unacceptable to me. A mom's gotta act like a mom. She has raised you and taken care of the family for at least the duration of your lifetime. And she's known you throughout your life. So, in practice, it should be quite hard to trick her or hide something from her or lure her into doing something she doesn't wanna do.