From the flashbacks at the end I got an hint as to what Tetsuo story is but I bet the manga goes deeper.
So, the prematurely aged kids, Akira and Tetsuo were all friends from the same orphan house right? And did they have powers or the power were created by the experiments?
If so why Tetsuo doesn't seem to remember the three aged kids were his friends/classmates? Also why he doesn't seem to remember who Akira is? And how he got into Kaneda's group if he was held at the clinic/hospital for tests and experiments? Did he run away? And why he develops his power better only later and not as a child?
The flashbacks containing Tetsuo and Kaneda are separate from the flashbacks containing the kids.
Tetsuo and Kaneda grew up together in the same children's home. You saw their first meeting as kids, where Kaneda took Tetsuo under his wing and protected him throughout life.
Tetsuo never had powers until he came into contact with Takashi on the bridge.
I think your confusion may stem from how Takashi kind of resembles a young Tetsuo in the flashbacks when the 'aged' kids were younger. Basically, the 'aged' kids were really children back 30 years prior to the events of the movie, along with Akira, as you see in flashbacks. Then the whole Akira thing happened.
About 20 years later, a young Tetsuo met a young Kaneda and became friends, which was shown in a different flashback.
The 'aged' kids are actually over 30 years old but have somehow remained in children's bodies, apparently mutated physically by their power or the government or something.
Either that or it's a reference to Hinduism where several of the protector or creator deities are either portrayed with blue skin or are closely associated with the color blue(as in the color of the sky or ocean). For example, Vishnu: