Commentary for Kiddushin 31:18
והא סימנין דאין להם קצבה וקתני אמר רב ספרא אין להם קצבה למעלה אבל יש להם קצבה
[others]. But what else does he omit, that he omits this?<span class="x" onmousemove="('comment',' It is reasonable that several items are omitted, but not just one.');"><sup>35</sup></span> - He omits her master's death.<span class="x" onmousemove="('comment',' For his heirs do not inherit her; infra 17b.');"><sup>36</sup></span> If it is on account of her master's death<span class="x" onmousemove="('comment',' That you say the Tanna also omits her father's death.');"><sup>37</sup></span> - that is no omission; since that applies to a male [slave] too, it is not taught. Then let it be taught!<span class="x" onmousemove="('comment',' That the maid is freed by her father's death, since nothing else is omitted.');"><sup>38</sup></span> - That which may be fixed is taught;<span class="x" onmousemove="('comment',' The term of six years and the proportionate repayment of the purchase price and the Jubilee are all fixed and ascertainable.');"><sup>39</sup></span> that which can not be fixed is not taught. But 'SIGNS', which are not fixed,<span class="x" onmousemove="('comment',' Not all women receive the evidences of puberty at the same age.');"><sup>40</sup></span> are nevertheless taught? - Said R'Safra: They are not fixed above, yet are fixed
Daf Shevui to Kiddushin
The answer is that while there is no latest date at which a sign can appear, there is an earliest date. If signs of puberty appear too early, they are not considered halakhically speaking to be signs of puberty.
There is a dispute about when a sign of puberty is considered to be a “sign” or just a “mole.” But all agree that after thirteen, any sign that appears on a boy is a sign of puberty.