But even with the new technology, and smaller units, the main recruits that the military goes for are young recruits. That's why they hit up high school and universities to try and get people to join. They still take older recruits, but militaries have always relied on younger ones.
The new technology means that they just have better control over the battlefield, and there is less fog of war, so they can conduct operations with more intel. They don't need as many people working the new technology, but at the end of the day, they still need soldiers, despite the size of the unit. It doesn't matter if the unit sizes are smaller (the US army is hurting for military right now, which is why they've lowered entry standards), the focus for recruiting age is still around 18.
Look at Norway....after highschool, students immediately go for 2 years military training (I think it's 2 years). It's not something you can delay, or say "i'll do it later..." After high school, you're doing military service. The reason it's straight out of high school is because the military relies on younger recruits, and it's easier to break patterns to train.
Technology doesn't affect the age of recruitment....it stays the same from 18-45. BUT, the military has always had a reliance on younger troops...ergo, if the age was moved up, it could possibly slow recruitment, which the military probably wouldn't like. The main focus has always been high schools and universities, although they take all ages from 18-45.
Also, the military wants a good amount of service time from soldiers in case of wars, and although they take people up to 45, the 45 year old won't have that long of optimal serving time, whereas a younger person who stays with the military will have a far longer amount of serving time.