However, you could get two years out of them - and if they've earned their spot with Championship performances, and likely have soft skills gained through experience you're all the better for it. I agree that if you've a 22 yo whom is in excellent form then they should be given the opportunity to play, but also if you've a 32 yo whom is in even better form and has put in the hard yards in first class cricket that they should be picked ahead of them.
I look at international cricket as the pinnacle of the sport, and that your best players should be selected, regardless of age. Shouldn't be attempting to let players learn the game in Test cricket. Also, one issue we've had with Root and Cook is the lack of captaincy experience - if the path to the England team is more hard fought, then they're likely to have captaincy experience, and if not certainly far greater tactical nous.
This. You want to win at any sport, you pick your best players.
Age is a secondary concern, unles you have two players of equal ability.
To turn a well-worn phrase on its head, if they're good enough, they're young enough.
Guys in their late 20s and early 30s are at the peak of their powers.
It's that perfect middle of a Venn diagram where honed experience & technique intersect with the physicality & reactions needed to play at that level.
You may only get 2-3 years out of them, but you'll stand more chance of winning every game than if you had a promising 20-year old.
Instead, let that promising 20-year old learn their game in First Class cricket, and earn their place in the Test team on ability. Not on a promise.
Take Ian Bell. If he's back in form and the best in the country, get him in. Worry about a replacement until he isn't.