Never mind the age gap, I would be very wary if either of my sons started going out with someone even their own age who was a parent at 21. If she planned for a child at the age of 19 then I think that's odd in itself in this day and age and I would wonder why she made that choice, and if it was an accident then I would also wonder whether she was mature enough to try and prevent another accidental pregnancy! I would be very wary at first until I got to know her and her circumstances.
Certainly in your case, if I were your ex I would be thinking "oh, Christ, even more shit could hit the fan here". Teen daughters need their dad to focus on them when it's their access time with him. Can you do that with a 21 year old girlfriend wanting your attention, a young baby around and potentially another accidental one on the way? Not a great mix, depending on the personalities involved.
Even if you were both doing your utmost to prevent an accidental pregnancy at this time, no doubt in a few years she would want a second baby. Could be just as your girls are going through A-levels. Again, not a great mix. Loads of added stress and added financial burden. You could be a granddad while still having a second family with teenagers at home. it wouldn't be for me but each to their own.
OP, you say her ex didn't treat her too well. Like others have said, make sure she's not just looking for Mr Reliable Father Figure. I'm not against big age gaps. It worked well for my parents (15 years) but neither of them had children from previous relationships to complicate matters. Or exes. I think he does seem more "old man"-ish to her now he's mid 80s but she knew that would happen eventually so guess she doesn't mind too much.
I do sometimes look at their relationship and think there was a power imbalance. She was a SAHM for most of her life and is more or less his carer now. Obviously they love each other but I often wonder if she has an regrets that he wasn't nearer her age.