I've been off the bike for 9 years, and just had double knee replacement. Getting back out there starting in the next few days - once I buy a bike. When I had to stop riding I had a 2003 Trek 5200 postal, which I truly loved and it killed me to sell. When I had to stop riding I was a total nut job, obsessed about speed, doing long rides at 25mph+ avg, making dangerous passes, and treating every ride as if it was a stage at the Tour. Needless to say, I made no real friends in 11 years of riding 4-8 times a week. Wasted years...

So I need a starter bike - and wouldn't you know it, my old Trek is available to purchase. So here is my three choices:
1) Buy my old Trek for $800, because I'm familiar with it and did so many insane things on it
2) Buy a $400 used KHS flite (with ultegra), just in case my knees hate cycling
3) Buy a slightly used Trek Madone or Specialized Allez for right at $1000 and move up to newer technology

I'm 72 and have never had a knee replacement but from what you're saying I'd suggest going to your local bike shop and get fitted to a new bike. Your body may have changed, your knees may want something you never thought of and the shop owner can get you on the right bike.

Climb on your old bike and ride it. If it feels good, buy it. If it feels horrid (which it may, your body HAS changed), leave it to the memories and go looking for a comfortable bike that allows you to ride the way you want to now, which won't be like you rode 9 years ago, even without the medical 'history'.

+1

It's not just a question of the frame and frame geometry, but of the gearing. I'm 55 and a year round commuter. In the last year or so I have "noticed" my knees more. My three bikes have different gearing and crank lengths.

I used to be a "masher" (higher gear, slow cadence) but now I find myself "spinning" more (lower gear, higher cadence). Of course YMMV.

I will also mention that I swam a mile a morning in my late 20s. TWo years after stopping I jumped in a pool and after less than three laps I had to stop and couldn't immediately raise my arms to climb out of the pool. (I was 31).

Find a bike you can ride comfortably now and work your way back up to something more sporty and aggressive, if that's something you feel like doing after getting back into some sort of cycling shape.

Buy the old Trek and mount it on the wall ... "Where it all started."

You might be able to rid it still, eventually .... but if it feels good to you immediately, you weren't trying hard enough back in the day

Still ... how many of us wish we had that first important bike in our lives?

The folks suggesting getting fit to a modern bike are right. But don't spend a grand on that first bike. Get a $400 bike and see if you still like it, and how your body reacts. Learn what frame you fit nowadays and how you like to ride. When you know how the new (older) body and new knees adapt, Then buy "The Bike" which you might ride for the next decade.

By the way ... you aren't allowed to post here past the first year unless you by at least two bikes. "N+1" is our religion.

Further ... you Must post pictures of whatever bikes you do buy. It's the law.

Many of us here don't even ride---we just buy bikes and post.