Deer selection is pretty simple.
What is in season.
Don’t shoot the really big strong animals with nice glossy coats etc. They are the ones to leave as breeding stock. Youngsters doing well should also be left.
Don’t shoot mothers with good strong youngsters in tow. - for above reason.
For meat I would look to shoot a mature buck but still relatively young but with a poor head and not the biggest of bodies
Or a similar aged female with no young.
If after a few outings you haven’t seen like the last two you then start justifying another beast.
How to tell age - experience helps, but pretty much the same as people or dogs.
Younger animals sprightly, move easily, head and years all up nicely. Pert hind quarters and in bucks good shoulders - ie they look as if they have been in the gym.
Older beasts - sloping back ends, longer noses, neck at more of an angle and middle age spread. And they tend to move more slowly and shuffle along.
Build up a picture of what they look like as they are walking about, and then who this translates to what they look like as you gralloch, skin and butcher them.
Have a watch of some of these videos- he has a lot of good information on aging bucks - many were tagged at birth so they have a very good idea of age.