Simple RAW for 5e 4: Smackdown v. RAW

[Koury]

A 20 No, if a form has a speed that is restricted at your current level, you can't turn into that form at all.

I'm assuming it says this somewhere?

The reason I ask is they could make a table about, say, a Dragon Shape ability that says the exact same things:

Level	Age	Color	Limitations
4	Wyrmling	White	No Casting, Flight or Breath Weapon
8	Young	Green	No Casting or Breath Weapon
12	Adult	Blue	No Casting
16	Ancient	Red

And it would seem clear to me that a level 8 character could turn into a Young White Dragon even though it has a breath weapon normally.