Firstborn is about five and half years old now. Secondborn is about nineteen months old - call it a year and a half. And even taking those ages into account, they're very different children.
The developmental stuff is pretty close. Firstborn is learning to read and do basic math, and doesn't seem to have any problems with either subject. Secondborn can say a handful of words, and just learned to hop in a way that gets both feet of the ground. He understands a startling amount of English, he just hasn't figured out how to produce the words himself. This is, I think, roughly where his older brother was at the same age.
But there are some noteworthy differences. Firstborn got his teeth relatively early; Secondborn's teeth are still coming in. Despite this, Secondborn has been eating grown-up food, exclusively, for months now. All the Stage 1 mush, the Stage 3 hot-dogs-in-slime that Firstborn was still eating at two-and-half years old? Secondborn won't touch them.
Secondborn is vastly more interested in vehicles than Firstborn ever was (or is now, for that matter). He rides the tricycles around at my parents' house, which Firstborn never did. And he's surprisingly adept at it: he can stop within two inches of my ankle, and he does. He plays with (and carries around) the Hotwheels cars - which were only mildly and briefly interesting to Firstborn.
In geek terms: Firstborn is the Transformers kid. He wants to be the giant robot that turns into other things. Secondborn is the Robotech kid: he wants to drive the giant robot that turns into other things.