Double the age is their weekly allowance. Age is base, which never varies. Second allotment depends on 1) getting assigned chores done without being reminded; 2) keeping up with homework; 3) playing by house rules (respectful speech, cleaning up and looking after their own stuff, being generally helpful.)
Second allotment can be docked for misbehavior or going against house rules.
Our allowance is generous, but we do not give money for movies, things they want, random snacks at school. It all comes out of their allowance. They do fine unless they start missing their second allotment and that's the idea.
Our oldest is 13, so gets $104/month.
We've been giving allowances for our kids since they turned 5. It teaches them how to value and manage money. It also meant we didn't buy random toys they lost interest in after 2 days. Funny how much more careful about buying things they are when it's their own money.
We gave them presents on birthdays and at Christmas. Everything else they had to buy themselves.