Déag is a noun, not an adjective. “Eight teenage boys playing” would require the sentence to change to either Tá ocht mbuachaill ina ndéaga ag imirt or Tá ochtar buachaill ina ndéaga ag imirt. (I’m surprised that the original sentence didn’t use the personal number form.)
I once saw the 31st (as in the date) written as ' an t-aontú lá déag is fiche' (the eleventh day and twenty!) You'll also hear things like 'trí scór go leith' to mean seventy (three twenties and half a twenty!) You gotta love it.
If you are saying, the boys are playing [football], then the verb "ag imirt" makes sense. But if the boys are children and they are just playing - i.e. not playing a particular sport or cards - then the verb should be "ag súgradh".