Is that actually true that you cannot to university after 25 in France? You could do Open University from the UK online; I think it is quite a credible program. It's open to almost anyone in the world as far as I know. If you are diligent, you can probably finish in 3 years even with a full-time job, but this depends on the person.

https://www.open.ac.uk/courses/computing-it/degrees