My house is only 20 years old but has a similar issue in the living room. There is a big bay window that juts out further than the foundation and I don't know what's under it for insulation but the living room is always 5 degrees colder than the rest of the house, unless it's windy then it's 10-15 degrees. Short of tearing all my siding and sheathing off and spray foaming the whole North wall, my next best option is more heat. I have a roll of PEX leftover from a job and a used 3 way zone valve from another job I'm going to use to build an underfloor radiant system in the joist spaces and tie it in with my OWB loop, just to see if it'll help. Won't cost me anything but a few hours labor so it ought to be worth a shot.
With that said I stopped by one of my renters on Monday. Their farmhouse was built in 1900 and has typical insulation for that era. New furnace and ductwork in 2014 when I bought it, heated pretty well for what it was but still had drafty windows and the like. They've been there 3 years and the old masonite siding was garbage, so this spring I gave them the ultimatum that I'd paint that crap siding and polish the turd a little bit and rent would stay the same OR I'd put new siding and fanfold on the outside with new windows and doors all around for an additional $150/mo if they signed a new 2 year lease. They wanted option B, so I had the work done which was completed mid-October. Went over there Monday to sign the lease and asked if it was any less drafty in there now, the lady sent me home with a pile of baked goods she was so happy lol. The guy said the furnace runs almost never now, and apparently they shut the thermostat OFF every day (for some reason, I don't think they understand how thermostats work) and only turn it on when they get home at night. It was 72 degrees in the house while I was there and 16 degrees outside, furnace hadn't run in 14 hours. Amazing what some good windows and a little insulation will do!