I was thinking the same thing, I think they perform so well even at this age is because playing in the IPL there isn't as much pressure as playing for your country so they can go out there and play their natural game where as playing for their country they might not be able to do that. Back in the day, players were retiring at these ages of 37 to 40 but now times have changed and players are retiring a lot earlier. So even at these ages they are still in good form.
T20 is a very short format and that may be one of the reasons. A T20 match lasts just 3 hours, compared to 8 hours for an ODI or up to 40 hours for a test matches. So it is more suitable for those players above 35 years of age, like Watson and Gayle.
It does not all about that...its about class talent power and experience.. and Gayle and Watson are one of the best players of all time specially Gayle he is the best batsman of all time in the t20 format of the game and still he is showing he is the voss