It's a personal choice...Why not? My Sebright x Old English Game Bantam pullet is doing a great job of mothering her chicks.
Pullets aren't yet at their fully mature body weight and often egg size even and they they are just as hormonal and still changing as cockerels even if they are a tad less obvious about it.
It's like the difference between a 14 year old giving birth and raising a baby verses a 24 year old. Women are different than teenagers. Their bodies and mind are far better equipped for the task at hand. And I can speak from experience since I had my son when I was 16 and my daughter at 26.
I also don't hatch very young pullet eggs and they must have been laying for several months minimum before I consider it.
And I let my boys boys mature separate from my gals before they ever get any sort of conjugal visits to spread their seed. Gives them a little time to cool their jets and I get to see how their attitudes develop.
I'm not saying pullets can't successfully raise chicks... what I'm saying is that I won't allow a STILL growing to bird to sit and lose condition on my watch. AND I think they are more and better equipped once they matured as well.
If she is indeed broody she will VERY likely go broody again in the future... since you said you weren't currently ready for chicks. Another thing I like to do is adopt feed store chicks to broody hens so I potentially won't have to deal with any cockerels.Ive wanted a broody girl so bad that I went and spent my life savings on silkies lol!

Ugh... My Silkie ladies have got broody hormones to spare! And will sit and raise chicks back to back to back through out the year if I let them.
