It's all about aging - not a popular subject, I know, but awareness helps. I have attended at least 4 fitness courses, including Pilates, focused on the older adult, with course content put together by youngsters, and taught by youngsters. None of them have a clue about being older, basically.
Over the years of teaching Pilates, I have noticed that even the skinny ones developing a spare tyre. This is a combination of loss of muscle tone and collapse of connective tissue: that wrinkly, lumpy stuff that used to be smooth! I work daily at maintaining muscle tone, but nothing can be done about connective tissue collapse. So although I do a lot of abdominal exercises - none of them sit ups ( a waste of time, ineffective and inappropriate for older people, and a complete no-no for those with osteoporosis) and I do have a strong core, my waist is ever-widening.
Another factor is that you cannot spot reduce: the only way is to lose weight, so you will lose the weight noticeably from your face: that's life! Really the only way to lose fat, besides watching food intake, is with cardiovascular exercise.
Remember we are all different, and our fat distribution is different. Another age factor is the distribution of fat changes as you age: if you had not much of waist in your youth, ie apple shape, post-menopause the fat will sit around your waist.
As for ab separation, there is still a well-known fitness/Pilates instructor, post three children, who has never managed to get them to meet again, as it is connective tissue, not muscle that separate. The exercise I recommend people with this problem, and anyone with a large ribcage, which also happens with childbirth, is seated spine twist which you can check out on Youtube.
There are loads of inner core exercises that can be done in Pilates without a single ab curl, or a plank.