They lose their age in SL apart from Youth players. So an age I player will lose 1 SL, age II will lose 2 SL etc.

Ta

Here's the rules snippet:

AN AGE. Age SBY is a schoolboy (the youngest player) , Age APP an apprentice (one year older), then Age I (one year older again), II,III,IV and so on until players retire. At the end of each season, each player will age one year, and will lose skill levels (see below) equal to his new age. Youth players, however, do not lose levels on ageing.

Basically youth players (SBY/APP) do not lose ANY SL as they age to APP/I respectively.

Everyone else loses SL equal to their NEW age.  So a II/10 player becomes a III/7.  Hope that clears it up.  

This is why it's VERY important to get your Age IV players to SL 7. If the Age IV players are no at least SL 7, they will not survive aging, and you will not be able convert them to BRBs next season.

My bad at getting the rule wrong, especially as I know you need a IV/7 to age to V/2

Losing skills...

New age, or current age ?

I saw different mentions I belief.

losing skills. i think it is meant to be new age, but I'm sure someone will correct  me if I'm wrong!

New age.

A I/10 becomes a II/8.

A IV/6 becomes a V/1 (and gets auto-retired).

A IV/7 becomes a V/2.

A III/10 becomes a IV/6.