Okay, so - hear me out here - Mikey is actually the oldest. He doesn't occupy the space of the eldest child in the family, he's still the youngest in that regard, but he's been alive the longest and Splinter just didn't think about it when he assigned their birthdays/ages. Raph was the biggest, so Splinter assumed he was the oldest, but he was only a tiny fraction of his full-grown size when mutated. Mikey was the smallest, but the closest to full-grown. And Donnie and Leo were about the same.













