I believe that the DG in my 150 is original since 1975, and still works perfectly fine. I did have to replace a gyro bearing in the AH about 25 years ago, but no problems since. Most GA electric turn co ordinators are a different matter, they have a finite life, as the motor for the gyroscope has brushes, which wear. When they wear, they also fill the inner workings of the instrument with dust. When I bought my 150 in '87, I replaced the factory turn co ordinator with a brushless one, and no problem for 3000 hours so far. It's worth noting that in most GA SEP planes, the AH and DG are vacuum, where the T&B or turn co ordinator is electric, so as to meet the requirement that there be two distinct power sources for gyro flight instruments. The newest electronic gyro replacement instruments get around this with internal batteries.
The only real problem with very old ('60's and earlier) instruments is that some had radium faces (so they would glow for night flight). That radium is now a hazard if inhaled (as dust) or ingested (as particles on your hands). Though you're allowed to leave these instruments in service if they're in the panel, nearly no instrument shop is allowed to work on them, and DIY people most certainly should not. When this came under regulation, I went through my, and two other friends stores of instruments, and disposed of the suspect instruments to the proper radioactive materials disposal. The owner of the instrument shop I used to work for died of cancer related to radium ingestion, so this should not be taken lightly. If in doubt, never disassemble an instrument, and don't put old ones on display on your mantle, where the kids can get at them.