Even though there are several, identifiable patterns within the genres of ‘cross-dressing, those patterns often reflect the person’s individuality as much they have in common with others.
In other words, it’s fairly simple but very complex.
For example, the age at which one first cross-dresses is indicative although unreliable as opportunity is a key factor and one may not have that opportunity at an early age. Also, any early occasions of wishing to be feminine may have met with resistance.
Even so, that you began at age 13 I would consider indicative as it lies within one of several age ranges commonly reported describe relevant behaviour.
The question that follows is, “What made you want to be feminine?” That is, your age at the time suggests specific motives, so can those motives be verified?