Hmmm well what are the perks of the youth ?
Blank sleight
The developer probably did not work in to many other companies so his mind is still open about a lot of things.
Spirit
If he/she is young, she's probably very motivated to get things done and you can use that energy to change and introduce new things or question old thing.
Reflection
Since she/he could be spirited and have no clue he/she probably will ask a lot of questions. The guys who hired my girlfriend as junior, hated and loved her for that. She asked them everything about their code while pair programming .... everything Gg.
Productions slows down a little, but the integrity and the knowledge increases. The trick is to keep the balance. For example you should iterate every two day or such. Since it's about learning from the whole team.
Maybe you could even Mob-Program if you like :)
New Knowlege
there is this "famous" quote:
if you aren’t a liberal when you’re young, you have no heart, but if you aren’t a middle-aged conservative, you have no head.
Which applies to our job as well as a young dev we don't mind breaking changes "we have to rebuild" -> @sandeep did post a lovely article about the balance of engineers which represents the different views. younger ones have less mistakes in their history so the tend to be less scared of change / failure which often starts with the words "you just have to..." a quote which send a shiver down a lot of seniors spines. We know that the devil is in the detail.
Still it helps us to reassess old decisions and maybe break our old habits. I tend to let them fix "mistakes"/"broken windows" so they can improve things without breaking major parts of the application.
And don't forget we did not learn all the things they did, I learned C in highschool now they learn java. Such things tend to bring some new perspectives.
These are the first things that came to mind. I hope you can take away something useful from it.
Edit: ;D i misread i was going for "of" not "for" .... anyhow ... i will leave the comment out of historical reasons ..... :)