Hey, i've recently bought a male suggie from a well know pet store, in Portugal. When i asked how old the suggie was, they only said that they first got him for about 5months. But he's quite big already, yet, he still looks pretty young
Is there anything that i could use to confirm his age, like his tail size, etc.?

Well if he is un-neutered than you may be able to tell by the bald spot on his head. (Does he have one?) Though the age of maturity for them can vary as well... my little boy started getting a bit of a bald spot around 4 - 4.5 months, whereas some will not get it til 8+ months.

By the time they are 5 months though, they are just about as long as they may ever get. They tend to grow 'out' from there. At 5 months, my little boy was almost as big as his parents.
Your best bet would be to ask the pet shop what his OOP date was, assuming they have it. But the 5 month mark is definitely when the line gets difficult to draw when it comes to gauging age by size.

Do you have a picture?

A vet experienced in gliders may be able to give you an approximate age. Like glidermommy said after 5 months it is very hard to guess an age.

Yeah, he's un-neutered and already got a bald spot, maybe 1,2cm diameter.