There were certain regiments that took smaller recruits like the one in Nottingham. I suppose they got desperate as the war went on. Not all older ones stayed at home. My grandfather was about 28 and not passed as fit for active service because of his deafness but was posted to France as a quarryman.
My other grandfather also a bit older was posted back to England because they were running short of electricians to run the trams. In Sheffield they blocked the government conscripting steel workers because they said you can't replace lifetime skills with some young lad or woman and they won the argument.