Is being an older virgin a problem for woman if marrying late? Is older vagina not stretchy enough?

Not a problem. The vagina can accomodate sexual intercourse with no problem in older age and 50 years is not that old really. A 50 year old will not be able to have children, but there are no limits to sexual activity in that age group.
Never too late! If you are not menopausal, it makes no difference what age you are. If you are menopausal, you might need estrogen to normalize the skin lining the vagina.
No not a problem. If you are menopausal it could make things a bit difficult, a premarital gyn exam and possible estrogen cream to help with vaginal elasticity may be indicated in that case.