User blog comment:Piyush Banerjee/Shin DxD 2 discussion/@comment-2208155-20181223125232/@comment-37165251-20181223160946

@Grandpiece We knoe that Sirzechs was able to enter into his True Form with 60-70 years. But we dont know when he was first able todo so, only that even with 17 he was already incredible Powerful (he could have effortless kill the Beelzebub Descendant that bullied the Oriax Heir if he wanted to).

So we don´t know how Long Millicas needs to reach his max Potential...but you see how fast Rias comes in two years.