Mix them up! I would suggest saving most of the 70s for later in your prep, but for example if you do 2 PTs/wk, do one from the 40s, one from the 50s, and the next week do one from 50s and one from 60s. Variety is good.
It's not business school.. There is relatively little to no age discrimination. If you've got the numbers and other qualifications then you're good to go. There are people in their 60s and 70s that get into the T14.