I actually think thats not Selim, more of a younger Bradley since they're all kids and such and I'm betting the artist based the younger version on the idea that Selim was supposedly the son of King Bradley so hence the "Like father like son"
I think it's "reversed ages." Like all the homunculus that are older in the series are younger here and vise versa. Which would me it easy to understand why it's Selim.
Actually, Selim was the oldest Homunculus. He looked the youngest, though. Gluttony was one of the youngest, so that's why he's a baby. Not sure about Sloth's age, and Wrath is the youngest, though looked the oldest. Artist just drew them in proportion with who actually came first, next, etc.
