Steven UnIVerse: 4shadowing

[John Cribati]

I'm 90% certain that So Many Birthdays was his shapeshifting abilities being affected by his emotional state. And in Steven's Birthday, he wasn't even aging himself, just stretching himself out (he loses control for a split second and his arm shrinks, not his whole body at once). He just reverted to a baby form because he used too much energy.