I need to install a toilet roll holder, but I blank out when
deciding where on the wall it should go.

it would obviously be used by many friends and family, young and old.

I don't think there is any "rule" for this, it's mostly a matter of
preference, ease of access, and available mounting locations.

Too far back and a larger person will have to twist around to access the
toilet paper.

Too low, too high, or too far forward, and small children may not be able
to reach the paper without leaning or stretching.

It also depends on how cramped the area around the toilet is. You don't
want to be bumping into the toilet paper roll as you're sitting down or
standing up. And, you don't want to be bumping it with your elbows while
you're reading a magazine or something.

Also consider the closer the roll is to the toilet, the more likely it will
absorb "splatter" and "splashing" from the males of the species...

In any case, the holder in our main bath is mounted to the side of the
lavatory cabinet, and is centered about 18" from the back wall, and 24"
from the floor. The holder in our master bath is mounted on a side wall,
36" from the back wall, and 32" from the floor. Both are quite accessable
and comfortable, so you can see there is a lot of flexibility in the
mounting location.

Sit down on the seat, and hold the roll where you think it is comfortable.
Then have one or two other family members do the same thing. Pick a spot
somewhere in the middle of those locations and screw it to the wall.

Anthony