Let's just say Sparx found the garden of maturity, which magically makes living organisms into adults, so he got Spyro and Cynder to go there, and they started growing up, Sparx convinced them he has magic powers to turn ppl into adults adding his glowing to make it more convincing while touching thep purple dragons, Spyro seems to be falling for it, Cynder, not so much.... it's the flowers doing...
From working for Malefor Cynder got to see many places so she knew of this garden, poor gullible Spyro....

I made Sparx more dragon like also, as you can see from the other lineart. LOL
http://isapphirus.deviantart.com/art/A-Spark-of-Maturity-WIP-527189518