For the age of goalkeepers, David de Gea's current age can still be used for a few more years, but with a note that he will be able to give his best in his game. Indeed, young players will give a very long time, especially in the position of goalkeeper will usually be difficult to replace.
Seeing from the last performance when he was still in the Manachester United uniform, actually his game was not disappointing, it's just that he didn't get the trust of the coach which eventually made him have to leave. In fact, his replacement players who have 6 years of age below him are also unreliable now.
Any club that is set to sign Da Gea should have him as a short term project. I am not a fan of signing old players for a long term purposes. I have heard some news about David having some eyes problems requiring surgery but at a time I couldn't read the story anywhere again. Was it a fallacy or an intentional propaganda?
Yes, I also agree on that, that when a plyer is over 30 years old then that player cannot be categorized for a very long time, maybe there are players who can last until they reach 40 years old for example, but not many players can do that.
This can be an option when they find it difficult to recruit younger players. Within a season or two, they can use that to target which young players they will bring in to become long-term projects that the club can use.
I just heard about De Gea's eye, and if that happens it will be a bad signal for his career if he still wants to play.