My bike has 15,000 miles. After several chains, the newest one I just put on skips a bit under load. Chain rings are original, never been changed. Cassette has about 10k on cogs 1-3, and 5k on cogs 4-10. It's a mutt cassette I put together to get the ratios I wanted. I noticed skipping in the small and middle chainring a few times today. Everything seems to be adjusted well when shifting. I assume cogs or chain rings are just old and worn, but I can't really tell from looking. Can you guys please give me an opinion on what I should replace? Thanks! Certainly no complaints about getting 15k miles out of anything.

Based on mileage I would say you need a new cassette and chain.

Chainrings should last a lot longer than 15K miles in my experience.

Sounds right.

If the current chain is new, then likely just a new cassette is in order. 15k is a decent life for a cassette. Cassettes and chains wear together so if you don't change your chains before you completely wear them out you can shorten the life of your cassette drastically. Once a cassette skips and shifts poorly with a new chain it's toast and continuing to use it will shorten the life of your new chain.

I'll try the cassette first as you all recommend, thanks. The chain is brand new. The cassette is not. I've been a good boy and changed the chain whenever the gauge says to, but it's been several chains now. So, I suppose nothing lasts forever. Thanks!

The same thing happened to me about a month apart on both of my road bike. Brand new chains skipped. In each case, I replaced the cassette, kept the new chains on, and the problem went away.