When it comes to extracurriculars, our experience has generally been that the instructors are very flexible about placement. A big part of this is DD is extremely attentive and on-task at all times. This makes a big impression on the instructors.
DD had a particular dance teacher at ages 4-5. This teacher approached us with the idea of bumping her up to the 6-8 group on her own.
DD started guitar lessons at 6. The teacher had never had a student that young, but he decided to give it a try.
DD at 7 did a trial at both drama classes for different age groups, 5-7 and 8-up. Afterwards, she asked DW to sign her up for the older group. The request was made to the teacher, who responded, "Sure! She certainly seems mature enough."
Now if only her actual schools would see what everyone else sees...