Obviously I can't add or subtract. That year makes a big difference actually.Zverev is 3 1/2 years younger than Thiem actually.
For a proper Apples to Apples comparison consider that when Thiem was the age that Zverev is right now (i.e. 21 years and 1 months) he was ranked outside of the top 40 and had been past the second round of a major once.
I stubbornly think that Kyrgios could be better than both of them, but I may have to abandon that fantasy soon. It is getting pretty untenable.