It's important to recognise that the average age of a WWE star did go down over the course of the 2000's but even now we're nowhere near what it was like in the 80's and 90's where most talents who came onto the roster were mid-late 30's minimum and many were far older like Harley, Andre, Flair, Slaughter and Shiek.
The end of the Attitude era saw a lot of young talent break through in a short space of time, Matt & Jeff, Edge and Christian, and the class of 2002 from OVW that had Lesnar, Cena, Orton, Benjamin - indeed Batista was the oldest of them by quite a few years.
It was easier to pick talent up younger then, as the indy scene was vibrant, kids like Edge started in Border City as Sexton Hardcastle, you had Smokey Mountain, USWA/Memphis, All Pro, for a while Shawn's TCW and of course ECW who would all take chances on younger talents, knowing that they would eventually leave, but make their reputations for them... Chris Jericho was still relatively young when he made the WWF, but had been seasoned all over the world so his young years weren't even noticed by WCW or WWF.
In the 80's it was impossible to get into the biz unless you'd been introduced by a wrestler, who was putting their rep on you by doing so. So there was a lot of hazing and paying dues, even uber talented guys like Ted DiBiase, Curt Hennig and Rick Martel had 2 stints in the WWF, one where they were literally kids, blue chippers or jobbers and their 2nd more prominent run when their "dues had been paid". 87-88 you got some late 20's guys coming in like Rick Rude, Bossman and Earthquake Ykozuna but they had seasonng elsewhere or unique selling points (Quake being a sumo for example)
Today that variety and seasoning isn't there because to get onto the WWE's main roster, even the acknowledged best indy wrestlers have to go through developmental and NXT. The indy's that are out there worry more about getting a "name" for their show than letting a kid become one, it's all economics. There is no "dues" that can be paid as there is no real proving ground bar time served in NXT and even then it's sink or swim. Guys like Chares Wright or Glen Jacobs wouldn't survive today's WWE developmental system as they had a few gimmicks before they found one that stuck. Today Unabomb would get Jacobs signed, Yankem bombing would get him sent back to NXT at best and Fake Diesel would have seen him fired... Kane would never have existed.
A lot of the names you have mentioned have spent a lot of time in developmental. Curtis Axel (scarily it's now 2nd nature to call him that) has been in developmental for 5 years, so he was only 30 when Nexus hit... Ryback is the classic case of a guy who has literally spent 6 or 7 years in developmental for the WWE so him being 33 or whatever now is not an issue, they knew what they were getting and have spent (or wasted) several years trying to get him to where he is.
Some guys like Wade Barrett will come to wrestling late, that in itself is no bad thing. Booker T. was a late bloomer, so was DDP.
Where it will become an issue though is the career length projection. WWE is steering away from older wrestlers after Lawler. Sting may be an exception but very soon a lot of big names will be approaching (or if you believe age doctoring) may have done already like Taker. Those guys will rapidly start to vanish and you'll be replacing them with guys not that much younger...
Also now guys have seen the Jericho/Taker/Brock schedule or guys like Rocky who got out early and with all his faculties, copuled with the fact they can make the money quicker in WWE than ever then you will get short careers. Much as I think Barrett is a great resource, his recent interview saying he wants to go till 40 was a major fuckup... Why push a guy you know wants out in 5 years over a guy who may give you 7-10 years on top? Someone like Randy Orton, while still having a lot of time on his contract could take that 3rd Wellness strike and the year off, knowing that he has been on top for 8 years and could come back and make it 10 without too much trouble. Someone like Bryan could after 5 years say "you know what? I've made millions here, I'm the most famous guy here now, but I wanna wrestle" and go off to Japan or set his own promotion up...
Someone like Roman Reigns or Dean Ambrose could hit the big time and get another offer from Hollywood or somewhere... guys who are older generally have families and make different decisions, they will work hard to get there and will decide for themselves when to leave. Look at Tyler Reks...sure he wasn't main event but he'd made enough money to keep his family comfortable and had a good enough fallback that he didn't NEED to wrestle again. In the 80's and 90's guys needed the road, needed the PPV royalties and needed to be on the show, asking to take time off like Ricky Steamboat did was almost a sin...today you get Ricardo saying of his suspension "It's 30 days I can give to charity and my family".
The answer of course is you need both, but the WWE has always led with this kind of decision, they invest the top push and spots in guys there for the long term unless your name is so big, like Rock, Hogan or Brock that a short term run works, or you can do a lot of business in a short time like Chris Jericho and no doubt Batista will when he inevitably returns to promote the Marvel movie.
The other factor to consider is that WWF is nearly exclusively hiring guys with college degrees now, JR constantly mentions it in his blog - so inherently the earliest WWE will look at a talent is 23-24 unless they are extremely talented or 2nd/3rd generation. Guys like Harry Smith had been wrestling professionally since 16, he even tagged with his father and he died a full 6 years before Harry made the WWE...he wasn't screwed by his age, just that he didn't have what they wanted at that time - he is a guy who will be back and now he goes by Davey Boy Jr. they have the gimmick all sorted for him - much like Axel being Perfect's kid.
