You can understand why babies and toddlers may do that - your body is the source of their comfort and happiness. It is normal for them to do it, although a more common comfort is sucking the thumb, for example.

As far as what age you may expect them to do it, this is entirely dependent on culture. Some cultures accept breastfeeding to quite an advanced age, others wean as early as possible - and habits like this track with weaning. 

If it makes you uncomfortable, you could think about dissuading the activity, especially if your culture will not accept it in public.

At the end of the day, they will stop at some point, so you can guide this point. And provide other comfort as needed.