You are never too old for a lot of things and colouring is one of them. Colouring for adults is a great way to relax and be creative at the same time.
Forget about the days of crayons! For an adult, colouring is very advanced with shading, gradients and blending all done with pencils and gel pens.
I personally love Johanna Basford and own a couple of her books. She has a great Facebook page.
There are numerous videos on Youtube on colouring ideas and techniques. Here is a fantastic example.
Please download my 4 free colouring pages and have a look at my pin board on that page with more free images.