Older cats are at higher risk for respitory infections, I'd suggest getting him checked out to be on the safe side. (The mucus could be a sign of a low level infection)
Ah, I forgot to mention. Kitty can catch the flu despite being vaccinated. Since it can be fatal, do be sure to get your old fella checked out.
Thanks Savas, so much for the flu vax doing its job, he's also an asthmatic too - this little guy has more problems than my ageing parents, lol.
Ah...to be a young kitten again. Then I could play and sleep, instead of just sleep and sleep. :)
I hope he feels better. Be sure to start adjusting his diet according to his needs as he gets older. Your vet should have some good suggestions based on his blood work.