It's basically the same as starting a youngster but better
. I honestly prefer starting older horses. Most recently I pretty much started a 6 year old mare a few months ago, she had been ridden but never started. She was just so quiet she let her previous owners children hop on without doing anything wrong. Before that, big memorable ones were a previously neglect case 12 year old mare, a dangerously aggressive 12 year old gelding ( i'm talking charging, biting to the point of stitches then when the guy went down rearing, stomping and breaking ribs dangerous), a just gelded 16 year old who was only just barley halter broke and the list goes on.
If the horse has had any decent handling they're pretty fun to start. You don't have the worry of growing bodies, you've usually got a better attention span, even though they're upstarted they've usually seen more, you don't have to redo anybody elses training etc. The only downfall for myself is the resale, people assume they've had more time under saddle than they do.
I've heard plenty people say it's more difficult and I call bull. If you go through all of your groundwork the same it's no different than starting a three year old.