She is not too young. Many first time moms begin their first year of maturity.
She is untried, so don't spend a lot on those eggs.
Favorelles tend to be on the lower side of the pecking order, if not the bottom, so if she is in the main coop, you'll likely need to isolate her from the rest of the flock so she isn't pushed off the nest. You can do that with creative partitions, or by trying to move her to a better location. She will need her own food, water, and ability to go somewhere to poop...broodies will stay on the nest pretty much 24x7 getting up once a day to eat/drink and relieve.
Forewarning, if you try to move her, many hens, especially first time broodies, do not like to brood in a different location and will try to get back to the original location or simply break brood. If you move her, move her at night with dud eggs to see if she will take the move. Then after she has resettled, in a day or so, place the fertile eggs under her.
Flock brooding can be very successful, but only if you have the flock dynamics for it....ie a very chill flock and polite gals that don't tromp on the nesting mom.
LofMc