It depends. My dad gave me his old Peugot UO8 steel framed bike from the early 70s, and I was all excited to try and fix it up. The frame is in good shape. The thing is, older bikes often have funny compatibility issues . . .
- If I want to change the pedals, I can't. The cranks are french threaded, which is non-standard.
- If I want to change the cranks for something lighter and that can accept regular pedals, I can't. The bottom bracket is french threaded . . . and there's only one compatible bottom bracket unit that I can find which costs 50$!
- The stem that's currently on it is an AVA stem . . . and apparently they occasionally break off mid-ride. However, it's a non-standard size stem . . . so there would be some bodging with sandpaper if I wanted to replace it with a newer one.
- The wheels currently on the bike are chromed steel . . . and they downright suck for braking power. They're also not 700C, which means I can't just buy a standard bike wheel at the store and replace them.
- The frame isn't designed for normal modern road bike spacing for the rear hub. It might be possible to mechanically spread the rear triangle to accommodate this, but I could ruin the frame doing this.
- The derailleur hangar is not designed for regular Shimano stuff, so I'd have to do some steel grinding and hope for the best if I wanted to replace the damaged old simplex derailleur that's currently there.
- The shift levers are damaged . . . but there's no standard mount for downtube levers on the frame, Peugot decided to go with a proprietary clamp on thing for their crappy plastic levers.
- If I wanted to use bar end shifters on the bike, there are no brazed on cable stops . . . so I'd have to figure out some way of running and routing the cables properly.
- Oh yeah, non-standard saddle post diameter, and the seat post that's currently there doesn't work with normal saddles with rails underneath.
- The front derailleur doesn't work all that well either.
It's possible to fix all of these issues . . . but with old components and changing standards . . . there does come a point where it's cheaper to buy a new bike.