If we go with Siborne for 1500 on 18th June as one known and 870men on 11th September 1815. 630 disapear between the two dates.
Based on the Guard Artillery on 16th June they have 28 officers, and 702men with the army, 26 men in hospital, 2 officers and 32 men not available for service and 5 officers and 49men left in Paris in total 809 men and 35 officers. The Controle book of the regiment lists on 23rd June lists 338 men desrted and 17 killed. The Controle of the guard artillery runs to 7th November 1815. So the desertion of 630men from the grenadiers does not seem too unreasonable. The controle for the 1e Grenadiers could not be found nor a lot of the 1815 paper work at SHDDT. It may well not exist. The registre metricule of the Guard artillery could not be located either.
We dont know the demographics of these men who disapeared. we know the demographics of 25% of the men present in September, we can say something about these men, but applying it to the regiment as a whole? That is the issue.
I have admission details of somthing like 900 Grenadiers ( the same also for the grenadier cheval, chasseur cheval and pied, fusilier grenadiers) so I can say for definate about the demographics of the guard at earlier periods, as we have definate numbers of men and definate number of men taken into the regiment so % can be worked out.
From what we know of these men who served in 1815, we can say that Sergeant de Mauduits assertation of the grenadiers average age being 35 with a handful of men below 30 seems wrong based on the data I have. 200men does not sound like a handful. The guard in 1815 as in 1813 included men who had little or no combat experiance.