In WW1 a lot of older veterans were recalled. They did not go to the front, but they were used to train men particularly young recruits. So a medical might not exempt you completely from service - it may only exempt you from front line service with deployment elsewhere.
There are a huge number of logistics jobs - a huge percentage of older men in WW1 were put into labour corps which isn't particularly well known about largely because they weren't front line battalions so they weren't deemed important so records for these battalions were not kept in the same way. A lot of what these corps did is therefore lost to history. They essentially were soldiers with spades not guns though and they did a lot of the digging and maintenance of the support for the front lines. The efforts of getting food to the front and getting ammunition to the front was enormous but crucial. They were away from the front line but this doesn't mean they were always out of danger. One of my relatives was in the labour corps and got caught up in gas attack and was hospitalised. It affected his health for years after. Obviously it's changed a lot since WW1 but a failure of a medical isn't necessarily going to stop you being useful in a crisis.
Again reflecting on WW1, people know that a lot of boys lied about their age to go to war, but this was cracked down on by 1918. And this didn't stop very young lads from enlisting. They could enlist from age 16 or 17. They were then called up immediately on turning 18 for full training. If they changed their mind in this period they were screwed. I don't think much has changed in this respect in terms of the youngest you can effectively sign your life away.
Another relative was in a battalion that was actually a territorial battalion. A lot of the men who were in it at the outbreak of war were immediately put on standby the day war was declared and then formally called up for training the following day. Many of these men were in the late thirties or even four times with the officers being older. Again many of these men were important as they had skills others didn't have and they were crucial in training later recruits who signed up after the declaration of war. They were sent to France in early 1915.
I think you have to see moves like this as increasing our war readiness. It is an uneasy feeling to see plans like this. Tbh it's probably not a bad idea because the territorials in 1914 were caught off guard really and it would be better for the reserves to be ready than to do all this after the outbreak of war.
The thing is, if you had talked about this ten years ago it wouldnt feel so real or scary. The fact other countries are also doing the same, if not going much further, and the fact it feels foreboding is a reflection on international instability... The two go hand in hand.