I am on console (PS4) so no mods and I had animal aging turned off when my pregnant mare suddenly aged up. She is now trapped in a permanent pregnancy and I cannot age her down.
Edit By Crinrict: Added Tag to Title, Adjusted Title
The horse ageing up while pregnant is weird but the "animal ageing off" doesn't affect cats, dogs or horses, they age with the household settings for ageing so if you don't have ageing off for households too they will continue to age.
Having a pregnant elder horse, when pregnancy in itself suppose to stop ageing, definitely isn't suppose to be happening.
