In fact, previously Keylor Navas was loaned to Nottingham Forest and returned to PSG but then Keylor Navas contract also ended. But indeed, being 37 years old is certainly a factor in PSG no longer extending Keylor Navas contract. Therefore, with his status without a club at this moment, of course Keylor Navas is free to choose which team will be his next choice to play for next season.
But even so, I'm curious, are there no teams from the Saudi League interested in playing Keylor Navas? Because after all, if there is a team from the Saudi League that is willing to bring in Keylor Navas, then at least Keylor Navas will get a high salary before actually retiring. But indeed, because Monza seems to be a team that is interested in bringing in Keylor Navas, then maybe that would also be a good choice to provide the best results for Monza next season.
At Goalkeeper position 37 years old doesn't sound too bad, could be last years of his career at any decent club, and he isn't going to be playing for a title contender neither, so I think at least another 2 seasons is possible for him, would be very hard but not impossible to do. I think it's clear that we are going to see him do very badly if not given enough time, but if he gets enough training going and learn the defense then he can play decently.
A player at striker may have some issues for example, or a winger could have issues, 37 years old is hard for those positions, wing backs too. But when you are a goalkeeper then things could be easier, you lose some reflex, some agility, but you are not going to be tired after a game all that much, so fitness stands even at age of 40 for a goalkeeper, if he is better than any other 25 year old they could buy or get, then he is still the best chance they have.
Just look at Milan a few years ago, they literally had Ibrahimovic and Giroud at striker, both 35+ years old, one 40 years old. And that's striker, not like goalkeeper, very hard to continue to do after that age, and they still did it. Italy doesn't look age, they look at talent and price.