How much do my career prospects suffer if I start a mathematics/CS PhD. at the age of 27?

I was doing a phd in a different subject and that was going nowhere and I lost interest in everything that I was doing. But I always loved mathematics and during the dark days of my 3rd of my (former) PhD. I happened to meet some exceptional theoretical CS people who encouraged me a lot. One of them got me as a TA and I am starting afresh in this new field.

How optimistic should I be about a career ahead?

I want to be a researcher/faculty.