No matter what your conditions, a toilet can last 50 years. Parts are still available for Case and Standard Sanitary toilets that are 75 years old.
The 23 year time period makes it a toss up...because it is probably a 3.5 gpf toilet, which was a toilet that really wanted 5 gallons, and they changed the flush mechanism to give it 3.5.
I would not lose sleep if you decided to replace it. BUT, a little acid bowl cleaner and some tedious work with come pipe cleaners and toothbrushes, will proably have it working. Make sure that someone has not previously replaced the flapper with a 1.6 version...because now you are asking that puppy to do a trick it just cant' do.