could be age, but it also could be he is in a down position, (which is a vulnerable position) and dogs can get intimidated / don't like it when another dog (or person) stares at them. I know Masi does not appreciate other dogs giving her the 'stare' or 'stink eye'..
growling is a catch 22, I would rather have a growl as an alert to me that something is 'off', vs no growl and go in for an unexpected attack.
With that, again, a catch 22, you don't want them growling but you don't want them to not alert YOU that something is off, If mine does something like this, I tend to go with a "leave it"..and "leave it' to my dogs mean, stop, ignore, "I got it covered", mind your own business.
Danger Danger vom Kleinen Hain aka Masi
"Angel" Jakoda's Bewitchen Sami CD OA OAJ OAC NGC OJC RS-O GS-N JS-O TT HIC CGC
"Angel" Steinwald's Four x Four CGC HIC TT
Harmonyhill's Hy Jynx NA NAJ NAC NJC RS-N JS-N HIC
Jakoda's Jagged Edge