There are no rules, I don’t think, as long as one’s head is covered in a meaningful way. I wear a bandana or scarf instead of a mantilla, with a few slightly nicer silk ones for Sundays (I cover about 75% of the time outside of Mass). I put those on when I get dressed and wear them all day. As far as mantillas go, they’re kind of impractical outside of Mass, so when I wear one I put it on close to the start of Mass and take it off when I leave the chapel. My best way to keep them from slipping is to get an elastic headband the color of your hair and secure the mantilla to the headband with straight pins. Other women secure them with bobby pins or clips or just leave them loose. I’ve mostly seen black or brown mantillas on women with dark hair, and white or blue mantillas on women with lighter hair, but there’s no rule about it. I’ve also seen mantillas in colors like yellow and pink, especially when young ladies celebrate their quinceanera.
As far as age is concerned, I’ve always kind of figured that a girl should start wearing a veil after she takes her first communion, but I’ve seen toddlers with little doilies, which might be a good way to teach them the habit. Again, no hard and fast rule. 1 Cor 11 specifies “women,” which I assume would indicate at least reaching the age of reason, but there’s no reason a little girl shouldn’t wear one. Of course, if I had owned a mantilla when I was five, I never would have taken it off. You would have found me outside making mud pies in it. But I digress.
I love headcovering and get a lot out of it as a devotion, as I’m sure you will. Pax!
-S-