The article talked about looks. Of course early 20s (and arguably even younger for the John Smith crowd) would be the peak of looks. Or basically, that's as young as you can get without looking like jailbait. That's nothing new. The average age of Playmates of the month when they first pose: 22.4 years old. The average age of Playmates of the year is 23.5 (but remember the Playmate of the year is usually crowned in the middle of the following year so it's pretty close to a calendar year older than the average age). I couldn't find comprehensive data on Penthouse pets but the more recent ones are older (being mostly established porn stars), but still in their 20s.
By the time a girl gets in her mid 20s (so 24-25-26), her breasts usually start dropping (even if so slightly) if not earlier. They'll drop sooner if she's been wearing bras all the time (according to a 2013 French study). Then they'll plateau or very gradually decline for 10 years or so regardless of having children. The hands age quicker than the rest of the body as well, particularly with the veins and knuckles and the skin just get looser. The eyes start to develop bags and crows feet. The pores also seem larger and deeper than when she was the fresh faced girl next door in her early 20s or younger. Makeup becomes much more important; they are much less likely to pull off the natural look. Of course some girls hit the genetic lottery or just take very good care of themselves and look youthful into their 30s or even 40s.
As long as a girl doesn't really let herself go or get her body ravaged by drugs or self-harming, which is a big qualifier, or have kids, which can often drastically change a woman's body, if she's a near 10 at age 20 (which usually indicates great genes), she'll still be close to a ten at age 30. The little changes from aging 10 years isn't going to drop her rating much. But the girls that are 7s and 8s and have less natural gifts to work with, the drop after 10 years is probably bigger and much more noticeable. Or put another way, going from a 10 to a 9, the girl is still going to be very hot. Going from a 7 to a 5, she moves from above average to the lower side of average (5.5 being the median between 1-10). She'll almost certainly get work done and wear more makeup if she's depends on her looks for a living.
As for why 20 year olds don't do as well at strip clubs? Nothing to do with looks. Experience is probably the biggest reason. It takes thousands of repetitions and fine tuning to gain expertise at a skill. Some people might be naturals or gifted, others may be over achievers, but it's hard to get around years of practice and experience. Many of these girls don't give good dances, aren't as comfortable in their skin, haven't developed their social skills or flirtation and role playing as much and aren't as desperate or serious as the older dancers. They don't know how to close deals, or properly offer extras (if they even offer them). They probably don't handle rejection as well (even the youngest hottest strippers get rejected) and less likely to hustle or move on to any and every customer or they might even just wait for the customer. They're also easier to take advantage of in terms of remuneration.
They might be in school still (if they're under 23, that is quite likely as most of the young girls haven't completely dropped out or given up on school yet) and for some of the girls, this might be their first real job or first well paying job, so they might consider a few hundred a night a huge amount of money compared to no money or depending on their parents or the minimum or near minimum wage job they're qualified for and be quite satisfied with that. They likely don't have mortgages to pay, children to feed and support. Their biggest bill might be their car payment or tuition/student loans.
So if looks were all that mattered, then yes, early 20s is where it's at. Of course IRL and even the strip clubs, looks aren't everything.