Afaik, US carriers are in service for 50 years and of course the youngest B-52 is coming up to its 60th anniversary, so 55 years does not seem out of line.
More broadly, these subs operate on sustained long patrols, so very predictable stress patterns. Combined with their well above average maintenance and support, I'd see no reason they could not go on for much longer..
Probably system obsolescence is an economic issue, although I've not seen any solid information on that. The report some years back that the Minuteman silos use computers working with 8" floppy disks does give a little perspective though.