I'm 30. I don't remember everything, but I do know the basics, and most importantly I remember what it is and what problems it can solve.

Couple of decades.

Anyhow, the important part about math isn't the memorization. It's the process.

Geometry and trigonometry aren't that useful by themselves. Even Algebra is marginal. Probability and statistics are way more useful. However, all of it comes together when you hit calculus which is VERY useful.

Can I integrate from memory anymore? No. Can I solve a differential equation from memory? No.

I'm going to use Mathematica if it's a one off. I will bone up on the process if I'm hitting it too often.

However, I'm always doing Bayesian probability estimates for all manner of things in programming, business, manufacturing, etc.

I'm turning 29 in a couple weeks.