A true undecended testicle can come down any time from now until 2 years I believe. I think it is after 2 years that they look at intervention as leaving it undecended may cause fertility issues (may be before Im not certain as we dont have personal experience with these ones).

Testicles should have a certain amount of up and down but should be in the scrotum when warm.

My DS has retractile testicles which is different again and a variation of normal where they go up and down and sometimes leave the building entirely (thats where the concern lay) which can sometimes be mistaken for undecended testicles. We were told they should remain down by 7 or intervention may be necessary for the same reason.

Both undecended and retractile should be checked 6 monthly from what we have been told. Again, no personal experience with undecended testicles though. Both your Gp and MCHN should be able to tell you more information about what is standard practice in your area.