When can I build Legionary Cavalry?

[bippukt]

I am in my current campaign as Brutii (v 1.5). I have plopped down a pro-consul's palace. But when I right click on Hippodrome, it shows that I can't train legionary cavalry, only more experienced Equites. I think that when I played two years ago (v 1.0), I could train legionary cavalry from hippodromes. I am a bit confused here. What's the requirement for training legionary cavalry . The scipii are running away in military power and I need to build up my army ASAP.