The standards that aang had and the standards jinora had for becoming a master are vastly different because of their conditions. Tenzin also said she wasn't ready for her tattoo until the end of book 3 meaning she didn't get them cause of her mastery of the 36 forms but because of her actions vs zaheer, unlike aang who mastered 35 and made the air scooter. Aang was also stated to be superior to his teachers in Airbending by age 6(he was better than kids twice his age)
and when he was 10 was better than his own teachers.
He didn't get his tattoo until 12 but the standards for him were far higher and stricter.Jinora had to convince her dad that she was ready through unconventional means, You could definitely argue that aang was the youngest master ever even if he wasn't officially recognized as one.And aang didn’t ask for tattoos but jinora did around ten times.
So jinora is the youngest airbender who achieved tattoos she IS NOT the youngest airbending master