I think it is rubbish to say that it takes longer to teach an older horse manners! I have never found it any different to any age.
The important thing to remember is to set tight boundaries to start, be very firm, follow through and be consistent.
I am ver experienced and you cab be assured that a horse will know within seconds that I will not take any nonsense! I will firmly but fairly correct anything that was unwanted - every time. I reckon it takes them two days to know that I wil not be walked through, pulled around or any barging in the stable.