Most old people will have old computers > Google ads overload the use of RAM immediatelly
Yes, most us us run monster machines > try opening your forum, probably overloaded with Google ads, on your mum's computer > it might freeze up the all browser.
Sell the banner space to end users (businesses directly) > the site will not crash your mums computer and you will probably earn more money (the 2nd is a different topic).
Huge +1 for less categories