A similar thing happened to me when I was in my late teens. I lost a lot on the temples but kept a great density on top until I hit about 25/26.
Look at the hair you shed. Are the hair strands different thicknesses? This is a classic sign that some follicles are shrinking and producing finer and finer hair which will dwindle away to minute, practically invisible fluff that no longer grows.
If you can get on Regaine foam (or a cheap generic version), then that might be a good idea. If you can't afford it then I don't think it is the end of the world if you wait 2 or 3 years.