When some people say older they mean six, and some mean seventeen.
Has the horse been handled or given any experiences?
For example, a horse that has been turned out and never been haltered or led will be much more difficult to start than a broodmare that is brushed daily and blanketed, gets regular vetting and has trailered to various farms.
Basically, an older horse will be the same to start as a younger horse with similar experiences. If you start a horse at ten, in three years he will behave like a five year old horse that was started at two.
That is something people may
forget- an older horse will not gain experience any faster or behave more reliably just because of his age. He still requires a length of experience to become trustworthy.
The main difference if a horse is truly older is that physical limitations such as arthritis should be taken into consideration.
If you haven't started a horse before, you will probably need some assistance once you get past ground work.
Realize that the liberty work will be helpful for the horse getting to know your voice and body language. But it usually helps very minimally with ridden work.
A horse will not relate the sight and sound of you he is comfortable with to the feel of you on his back. There are no shortcuts to experience under saddle. Meaning, if something spooks him, he won't stop to think about if he trusts you. Instead, he will react based on how comfortable he is with the reactions of the rider, his experience with seat and rein cues, and how they are applied.
At fifteen, he could have arthritis or other physical problems to keep in mind if he shows resistance.