Normally, I have seen a trend that average age of every such PLs & SLs gradually reduces for first few years, then becomes more or less steady. Main reason is that, when such leagues start, for commercial values, brands/owners try to hire reputed star faces. Besides, at the start often team management also are not sure about the talent pool from domestic circuits - therefore they tend to be comfortable with known faces, seasoned FC cricketers who had been performing in domestics for a decade. Once the team is settled and brand is established, franchises tends to hire younger talents - apart from other reasons, it helps to phase out oldies and manage the budget as well. I can give a simple example is that - next BPL, Afif will be a hot shot, and he'll make one of Nasir or Mahmudullah uncomfortable; this year about 8 or 9 players played in BPL who were still eligible for U19 WC, and this number should grow next year.
PSL is in 3rd season, I expected the average age to go down considerably, because of the scouting network; but it's not started yet to be honest. Still, most of the top tier contracts given to seniors and there is hardly any effort to bring youngsters in leadership. That's important as well to reduce the average age, because an aged Captain will always be more comfortable to have contemporary players around.
