Originally Posted by base2

10,000 easy miles is a lot different than 10,000 hard & neglected miles.

Some people have a favorite cog. This is the one most likely to skip under load. If you are someone who uses all your cogs, you'll get more miles.

Big gears (larger diameter) are under less tension, which makes for less friction & less wear. The smallest chainring is the most likely to wear out first unless it is a much stronger material than the others or used significantly less than the others. Chain-suck where the teeth are too tall to cleanly release the chain is the biggest tell-tale sign that a replacement is necessary.

I wouldn't change anything until a fresh or relatively unworn chain misbehaves.