This sentence is incorrect because the verb ser is being misused. This sentence must be in the imperfect tense because this tense talks about past actions, conditions, or events that occurred regularly or frequently or that were in progress in a moment in the past. So the correct conjugation of the verb ser must be era that matches the third person singular in the imperfect, not es that stands for the simple present. Finally, the correct sentence is:
<em>Cuando tu madre </em><em>era</em><em> joven, tenía el cabello largo.</em>
