Hello Natalie,
welcome to the forum
Hair loss is a normal part of getting old for a hamster and at 15 months old this is usually around the time when they do tend to lose their hair.
There are other possible causes, such as mites, as you have suggested - apparently (so i have heard) all hamsters have a very small amount of mites on them but as long as the hamster is not stressed and healhty and active and their cage is cleaned out regularly, the numbers are controlled and they never cause a problem, even though they are there! So as you can see, they dont actually 'get them' or 'catch them' from anywhere, they are already there, but when they start to cause a problem this is when they are noticed. So the reason that they may be causing a problem now could be another illness, or she could be stressed for some reason, or it could just be her age.
Another hair loss cause is Cushings, but this usually has other symptoms too, but i dont know much about this condition, so hopefully someone else will be able to tell you more about that.
At the moment though, i would just treat it as signs of her getting old, unless she has other symptoms, but please keep us posted
Tammy x