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When you join the army, you sign up for 4 years. You can leave at any time after this as long as you give 12 months' notice. If you want to stay you can extend your 4-year contract.
22 years is actually the maximum that a soldier is ordinarily allowed to serve and when your full pension is activated. But I appreciate I'm being a pedant here. In theory, as long as you meet fitness requirements, now days you are allowed to serve up to 60 years old or so, but that is at their discretion.
Stonehedge-
I had to check this because of family background and klaxons going off. Are we talking British army?
Both my dad and my brother joined under 18, it’s still the case you can join up at 16. I pedantically checked. You can start your app at 15 3/4 and need parental consent under 18.Which is nuts. Especially as at 16 my brother's rifle was as big as him. But that was a loooong time ago. And even I’m taller than the current standard issue rifle.
Chalfie
You join at 18 do 22 years and retire at 40 unless you become very senior. If you are infantry your knees retire before the age of 30.