Age range prediction according to browsing history
This problem is quite broad and I would like to add some of my suggestions. Refer to good source of Age wise browsing data, age-wise social behaviors and their preferences publicly available. Here are some datasource that you may refer and you will have to do extensive research on lot of factors to predict age based on browsing data.

Datasource 1: https://www.google.com/publicdata/directory

Datasource 2: https://www.ciser.cornell.edu/ASPs/datasource.asp

Refer to this paper : https://link.springer.com/chapter/10.1007/3-540-44934-5_3

I would definitely say that this problem doesn't seem to have any perfect solution, only thing we may do is to try our level best by various sources and implementations. Learning algorithms on public datasets on behavior, survey,  social and mental age specific activities etc can help as well. I have also attached a link to a paper on similar problem. I hope this helps to some extent.