See the thing is - the stage the player plays at is only a part of the equation. So to say that Suárez was only at Ajax at 22 is a bit disingenuous - because even at that level, Suárez's technique and desire and relentless workrate on the pitch (sometimes, that manifested itself in a cnutish way) stood out, way above the rest of the cast (I think it was Van Basten who compared him with Ibrahimović in terms of his impact on the team?) Of course, very few could have foreseen what he would eventually become at Liverpool and Barcelona, and the Eredivisie isn't comparable to the Premier League, but the raw tools were there in abundance as a forward, not just as a #9 (something I don't quite see with Lukaku). At approximately the age Lukaku is right now, Suárez was exceptional at the World Cup level too (Uruguay's best player after Forlán from a wide-ish position - and IMO, they could have even reached the final if he played against Holland). He was a lot more established than we are led to believe per the Liverpool unearthed an unheard of gem narrative. That said, I do agree with you in that Lukaku could achieve far more than was expected of him, we can never discount that possibility.
As to the kind of strikers or forwards we could sign, that's an interesting question. And a lot of it hinges on whether we manage to qualify for the Champions League (or not). If we do, then the potential addition of Mourinho (pls!
), and the money United can part with - should make for an interesting summer. IMO, Ibrahimović is the best option because while he might only be a short term addition, more crucially - he's still a Top 3-5 striker, and will provide an immediate impetus to the attack (the central piece that Martial and co. could rally around). Griezmann depends on the aforementioned CL qualification bit I reckon. He loves Atlético (apparently), and seems comfortable in La Liga, but Atlético's finances are still in a flux, and Simeone can already spoken of the possibility of letting Griezmann go. If push comes to shove, United might just be able to nab him. Dybala on the other hand - won't move, and neither will Lewandowski (if he does - it would be for an obscene amount of money to Real Madrid or summat - thought parting with €60m+ for a player like him would be more palatable).
And reg. the Dybala/ Lukaku age thing, that's another unfair argument IMO, because players don't progress in a linear fashion - some are fast out of the gates, other not so much - and Dybala moved all the way from South America too (transitioning to European leagues is extremely hard), and again - it's about the total skillset, instead of what the player has 'proven' - IMO, which is where Dybala sets himself apart. Infact, it kinda reminds me of the arguments I used to have with my Arsenal supporting friends back in the day - who were of the opinion that Reyes was better than Ronaldo because he was scoring more goals, and was more 'proven' at that moment in time - without considering their relative skillsets (not the greatest comparison to be fair, but it's the first one that sprung to mind).