You can actually improve cognitive function. Perform most of these tasks at least a few days out of every week, make a schedule.
>Learn a language (from French>Spanish>then Japanese, or start with Spanish; /r/languagelearning, /r/learnspanish, /r/learnjapanese, /r/french)
>Exercise (bodyweight fitness or weightlifting, swimming, bicycling, running; /fit/, /r/bodyweightfitness, /r/swimming, /r/bicycling, /r/running)
>Read (from the Oxford World's Classics list, not Harry Potter; /lit/)
>Write (can be a fictional story or a personal account; /r/writing)
>Learn a musical instrument (start piano or drum set, not guitar; /r/learnmusic, /r/drums, /r/piano)
>Meditate (also be stress free)
>Create (drawings and/or DIY projects; /diy/, /ic/, /r/learnart)
>Get 8 and a half hours of sleep (every day)
>Learn the Trivium and the Quadrivium (/sci/, /r/grammar, /r/rhetoric, /r/logic, /christian/, /r/philosophy, Khan Academy)
>Make friends (go to a game shop near you and find a group to play a tabletop or card game with; try to meet up at least once a week; /tg/, /r/rpg)
>Learn a skill (first aid, personal financing, house and car maintenance, cooking; /ck/, /r/personalfinance, /r/cookingforbeginners; /r/homeimprovement)
/freedu/, /edu/
It's going to be hard till you make them into habits but I guarantee you will become smart. Check out community colleges, they are great resources for these tasks but will cost money.