Not necessarily. My team does quite well on counter and we have Son and Kane who were probably not as fast as they used to be,,, but much quicker in mentality and thinking, with the experience to see the right openings which as youngsters, can be bery hard to see.
Juventus is a not exact example but yes in general the past 2 seasons have seen the oldest players score goals as counter in Serie A. I would not say they are worse in UCL though,,, Atalanta and Napoli both gave Liverpool such a hard time remember?
This doesn't mean Zlatan wasn't better when he was 28, he could be better when he was 28, but that is only compared to himself. Juventus has Ronaldo for example, still better than many other players in the world, sure he may not be as good as he used to be himself, but he that is only compared to himself. We have to remember this when we look at their age.