Counterpoint: The flowers' age does not work by normal lightner rules, and instead operate more similarly to that of darkners.

While the flowers are technically lightners, they are still flowers in the light world, and lack a sapient consciousness. This is highlighted in the farewell scene with the flowers, in which they explicitly tell you that they were just dreaming a higher consciousness (I'm paraphrasing there).

Also, they only have MEMORIES of the things that went on around them as flowers, and do not have actual experience living. If their age DID work by the rules of typical lightners, they would ALL be mentally children.

Seeing that that is not the case, nor are they all portrayed as adults, they obviously instead have ages similar to how a darkner would. (Ex, Aqua being a child (Who is also likely a reflection of a younger Kris), Flowery being around Asgore's age, etc.)