Hi,
Age doen't really matter for studying..It's your life and you should do what you wish..That's how I see it. Infact schools might consider it as a +ve point becos you should be having many years of experience in dentistry.
So are u practicing dentistry? if so which country?..All u have to do is get your credentials evaluated in US, take NBDE part1 & Toefl ASAP..& call schools get answers for your queries and APPLY ASAP