My experience with juniors is mixed. There were some who have learned quickly and eagerly, but there was also a fair number of those who didn't yet know what they don't know, so they thought they knew everything.
If you work with other experienced people you can in most cases rely on them not to do very stupid things. With juniors you don't have that luxury, and it takes more time to explain to them why what they did was wrong. It takes all kinds, I'm sure, but if what you're doing matters, I think the ratio should be something like one junior per two seniors.
Honestly, the name itself, senior programmers, is silly. I just call them programmers. You don't go to a senior doctor or a senior hairdresser, do you?
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