Personally, I rarely use a mounting block at six-foot-two-inches I have a fair reach, even for an older person. But an observation is that a lot of people struggle to keep their horse standing still while they mount from a block. What I have found is that horses simply outsmart us, they are often one step ahead in remembering patterns of behaviour.
We see that in the reining world where, the horse has drilled the patterns so many times, they can easily handle being bridleless. You might find that if you always follow the same route out on a trail ride your horse will toss its head or object to going a different route. Horses love routine and consistency, this is something that works both for us and against us.
Back to the mounting block – the rider gets their horse to side up to the block and stand while they prepare to mount, everything is good until a foot goes in the stirrup and the horse walks off before the rider is settled and ready. The problem is the routine, the horse is anticipating everything that the rider has set up and the horse knows very well that once a leg is over its going to be asked to move out (forward) from the block, he’s just getting ahead of the rider.
The rider can break the anticipation by asking the horse to back up, roll back or side pass away from the block. One of the things that horses rarely do, when left to their own devices, is back up more than one or two steps. If they do it on their own its usually followed by a roll back and moving out forward. If we make a routine of sitting still while we adjust our position in the saddle, then ask them to back up 3-4 steps before leaving the block they will anticipate the back-up, which they’ll be in no hurry to do and wait for us to get settled.
If you are trying to correct a problem be prepared to work at it for a few rides before a real big difference will be seen. In the meantime, if your horse starts to step away, gently pick up the reins and guide him back to where he started and finish your new routine.