I am 49 and I stopped using Mirena IUD. My period returned to normal for a few months but now, I just missed my last one. Is it the menopause? Is it true that the later you start menstruation, the later the onset of menopause?

The age of menarche (first period) has nothing to do with the age of menopause.  It is very possible you are in perimenopause and experiencing your first shift in hormone levels. 

You will most probably have some more periods, because they seldom just stop suddenly. 

Hormones tend to shift for a while before actually going for good.  I would suggest taking MenoSupport Complex from A.Vogel to help with your night sweats as well as to help to prepare you for the hormonal balance in post-menopause.