It wouldn't hurt to ask, but I would text her, so you don't put her on the spot, and she has time to think of a rejection excuse if she doesn't want your other child there. It may well be that she says no, because she wants the party for older lads, and it's nothing personal... And if you DO text her, it gives her a chance to think of a reason to say no. I think she will be very resentful if you put her on the spot.
I have 2 tales regarding this. One, a woman asked me if her daughter 'lucy' could come to my daughter's birthday party (some years ago when they were seven or eight -ish,) because her bff 'lana' was invited. My daughter had not invited lucy, and her mother said she must have forgotten to invite her.
So as I was put on the spot, I said (stupidly,) 'umm ok I guess she can come.' I went home and told my daughter, and she went mad. She said lucy is a nasty bossy little moo, and she can't stand her! And she said I must tell her mother she can't come! She was really upset. Her sister agreed and said I should have said no!
I had to tell the woman - the next day - that lucy couldn't come, as I had mentioned it to my daughter, and she had said lucy isn't in her friendship group. She said 'well she MUST be because Lana is, and she is Lucy's bff.' It caused such a huge stink. It rumbled on for weeks and weeks, and the mother gave me filthy looks for weeks, and got her mates to so the same!
Horrible!
So as I said, text the woman, don't ask her face to face. Give her a chance to think of a way to say no if she so wishes.
The other tale (a shorter one!) is a few times when my daughters had parties, we would also invite an older or younger sibling of one of the kids invited (up to about 5 years difference.) So the age difference is irrelevant imo. (Unless the main age is 5 and the younger one is 3 months old lol.) But if it's mostly 7-8 and a 12 y.o. wants to come, or mostly 11-ish and a 7 y.o. wants to come, then why not?
But yeah, do TEXT the woman and ask, don't ask her face to face. And don't take it personally - or be sniffy with her - if she says no.