So that way they all have 'em?

Its dependant on a lot of factors Jamie Madroxx aks Multiple Man had his abilities since birth as had Nightcrawler he was born with his physical mutation. Others develop their powers later in age usually in their teens such as Rogue, Cannonball, Magneto, Professor X, & Cyclops. Other deciding factors are puberty, stress such as the fight or flight response like Cannonball when his powers manifested when he was trapped in a mine shaft cave in.

Define what you mean.

Do you mean how long does it take for a mutation to develop in a comic book or how long it will take for us to have them in the real world.

I'm going to assume that you mean comics seeing as this was posted in the Marvel Mutants section, and the here mutations develop around the teenage years by a product of a gene mutation. Some mutants manifest powers at birth, but all have their powers around that time just dormant. If you are wondering how the other humans may develop mutant powers some will not. The Human race got here by wiping out or out-evolving the competition.