What age limits can a 40 year old guy set for his gf or partner?

If you're looking for a number I would say the lowest would be 35 and the highest would be 45, this is of course considering the matching is good, there's no telling since everyone developed and thinks different at different ages.

Firstly the mindset of Shopping for a suitable partner must be discontinued. None of these parameters really matter. Instead aim to first connect with a soul and interact and see if one is able to establish a connection and there is synergy between. Other things will fall in place.

18-130

18 is youngest, no upper limit

That’s up to the person.

Mental n emotional age ought to be the priority to lead a safe partnered life.

28-52

How about 18?