hmmm they are fantastical creatures.... they could be reptiles if you want them to be 
Why would you
want them to be reptiles?
Slaves to sunlight, sluggish in the cold, no flight (no reptiles fly).
Seems to me that
imagining them to be reptiles also requires you to imagine away a whole raft of reptilian characteristics that get in the way of how we believe a dragon should behave.
"let us slay the dragon...later, after sunset, when it will be asleep, and have no energy to move..."
"we offered the virgin to the dragon, but apparently it had already eaten that month, and wasn't hungry..."
I
personally imagine dragons to be creatures of magic, that don't eat (for sustenance), breath (for oxygen), or age.
But that's in MY WORLD!