Should I start my Personal Statement with "Since an early age/Since I was a Child"?

Yes and no. Although this a very easy and convenient way of starting a personal statement, it is also an extremely overused starting phrase, it will mean that admissions tutors will be less prone to find your personal statement remarkably original. However if the rest of the content is great they might overlook it. Overall I'd say that it is better to avoid starting personal statements like that.