actually rank reduction is very rare. even if so, usually will
be reduced to the rank of PC. is the ASP you got reduced to a
corporal a VSC?
because for us, usually for JOs, will be reduced to PC before being
dismissed, in accordance with the Police Force act. For senior
officers who get into trouble, they will be 'asked to resign'
altogether.
about your quesntion, though... i think can go beyond 55 for those
on pension scheme. cos on a case-by-case basis, they can be
re-employed. one year or two years, will be dependent on MPO lor.
If MPO and the relevant people upstairs have no issues, you can
keep getting re-employed. but the danger is that if anything
happens to you, you might not get your pension. or smth along those
lines.
for the newer people on permanent scheme, basically they can stay
till retirement. if not, work until whenever they wish.
but if you asked me, so long as not 'retired in the name of public
interest' can already lah. 
