I am very puzzled over this.
Only a small proportion of the 18 year olds would go into a full-time one-year military post, even according to their plans. It's not going to solve the military recruitment crisis or reverse the cuts.
As for the rest of it, as far as I'm aware, anyone including 18-year-olds, can already volunteer one weekend a month! If it's not compulsory and there's no penalty if you don't do it, then you can do it. Lots of universities have ties with voluntary organizations and lots of students volunteer already, but the key problem is most need jobs and can't take 1 in 4 weekends off.
Charging students £9k or more a year, then limiting their earning power by not allowing them to earn the full adult minimum wage and then further limiting it by not allowing them to work all weekends- this generation is being further shafted!
If they want to introduce an expansion of the Reserves, or set an example by volunteering one weekend in four themselves- crack on!