Regimental children could become fifers or other regimental band members when they reached the age of 14, then sign up as a soldier in the regiment at 16.
In a battalion of a regiment, each company had 2 tambours, they were usually men but in the later years I expect young boys became drummers in the later yearss 1813-15.
When the battalion was formed together, say in line of battle, all the tambours formed together behind the centre of the line.
The most important musicians were the drummers, they beat the orders in both battle and garrison. They beat the rythem of march, the charge, the retreat, the assembly etc.
The regimental bands were mostly for parade and ceremony but were utilised in battle at the Colonel's whim, for example the band of the 23e Regiment d'Infanterie de Ligne played La Chant du Depart as the French began their attack on St Amand at the Battle of Ligny 1815.