Oh, you were being ironic, eh. ?
I just assumed that you were an American.
Yep, even a drummer boy, and bear in mind, when someone enlisted, they had a choice of regiment/corps, at least they did until later in WW1 when manpower needs took preference over the wishes of recruits.
Today a recruit will enlist at their local recruitment office, and then be sent to train at wherever the training depot of his chosen unit is, much the same procedure as it was back then, except that back then he would have enlisted at the army barracks of whatever unit happened to be in his local area, so for example, a recruit in Glasgow might have enlisted to serve in the Royal Artillery, and have done so in an infantry regiment barracks in Glasgow, and then have been sent to Woolwich in London perhaps, to do his basic training in the Royal Artillery.
You keep calling him a drummer "boy", and perhaps he was a boy, you have only posted a picture of his hat !,...the minimalist approach.
... and certainly at least up to the late Victorian era boys were legally enlisted as musicians, and even in recent years 16 year olds, and I think even 15 year olds, with parental consent, could enlist for junior service in the British Army.
In WW1 the minimum age for adult service was 17 in the territorials, 18 in the ordinary army, but nobody was supposed to serve overseas until they were 19, but of course many thousands of underage youths both enlisted and served abroad at much younger ages,...his hat is typical 1902 pattern service dress, so he could have enlisted pre WW1, or post WW1.
There were adult drummers in the WW1 and pre and post WW1 era, and if a man served overseas in WW1 he was entitled to campaign medals, and if you go here, and search on the rank of drummer, you'll see lots of supposedly adult drummers...
http://www.nationalarchives.gov.uk/records/medal-index-cards-ww1.htm