Not really no. If you're suggesting that a 23 year old has an equal chance to decline than a 32 year old then I simply disagree.
No, not saying that. However, a 23 year old is not yet in his prime, so him reaching that peak is still a big question mark. Griezmann at 33 sounds like he is in his prime being at his career best.
If you get hold of a 23 year old of equal quality then jackpot, but a 23 year old is normally not at his peak and subsequently there is an expectation of growth. That expectation may be met, or may not be. In most cases, you would be hard pressed to find a 23 year old in a similar position for similar money as Griezmann. Now, if Griezmann's quality drops by 10% in the next two years, is he going to be worse than the then 25 year old almost at his peak, or expected to be almost at his peak?
This is why I find the discussion on age so one sided. If you took a 32 year old Modric at the time, many would argue against him because of age. He's 37 now? That's five brilliant years of a fantastic footballer who is still one of the best in the world - was the best midfielder in the world at 35. If you buy an almost class player at 20-23, you expect them to improve, but that's not always the case. Thus, you need to look at their current level, and you need to consider the drop that is bound to come from at 30+ player, but that needs to be seen in comparison to an expected rise in the level of the 20+ player.
If it's a two year contract for a small fee and he hits the ground running and still produces at 35, then it was a solid investment. RvP was a brilliant investment if only for that one great season. We would not have won the league without him. Re-sale value is an issue, but then United are not exactly the best in class in that department.