Thi took me a while to notice until i opened a horse shop on a server i play on, in both multiplayer and singleplayer, baby horses render as almost full grown, this goes as far as to show "99%" on Zyan's HUD when checking their age. I have removed every mod and even ran a clean version of mc with and with optifine, both did not have the bug, it was only when i loaded back into forge that it started happening again, I am using the latest build, and not the recommended build, after i post this i will be checking the recommended build to see if the bug exists there.
EDIT: After running the recommended build, the bug does occur as well...