To be fair, I'm not keen on anyone below 20 to take on a Predator. Stretching it to 18, depending of the character written is well done. After what The Predator has brought with Upgrade chasing after Roy, I'm not too keen on other stories of Predators chasing after kids again. That one comic with the 3D glasses was super silly as it is, but actually having it on the big screen? Yeah, I don't think so.
Besides that, yeah it really doesn't matter if early 20s or late 70s, long as you have the skill and the will, with a little bit amount of luck, there is a slim chance in succeeding taking down a Predator.