Yeah dolphins - dolphins have brains equitable to ours in size and complexity, and baby dolphins are almost like "little tiny full grown" ones, at least for learning curve purposes...
I don't know about cows being a fair comparison though, as hoofstock in general from deer (the tiniest antelopes to the biggest sable) have only one or two at a time due to herding conditions.
I'd still say that our dragons can grow darn near as big as they'd like to: dinosaur eggs containing baby dinos are only the size of my hand fingertip to heel - and to grow to a size of like 20 feet long from that is pretty impressive. Most of our dragons eggs, at least according to lots of our hatchings and the pernese books, are quite large - I would say ostrich-egg sized at the smallest. They've got room to develop.
And heck, why Avens would take so long? They've got to master magic and psionics, their culture is set up for it. Perhaps in their deep past they didn't take as long, but had less to do or learn.
.... Iiiiii.... dread making a chart like this for my dragons...
