Ma'arif al-Qur'an
Mufti Muhammad Shafiإِنِّي وَهَنَ الْعَظْمُ مِنِّي وَاشْتَعَلَ الرَّأْسُ شَيْبًا"Here I am that bones within me have turned feeble, and the head has flared up grey with old age." - 19:4Human body becomes weak with the aging of bones, because it is the bones which keep the body firm and upright. When bones lose their strength, the whole body turns weak.
The literal meaning of اشْتَعَالَ is a sudden burst of fire. Here the word is used as a simile for grey hair covering the head like a glow of fire.
Mention of one's needs in supplication is desirable
Here Sayyidna Zakariyya (علیہ السلام) before making his supplication, mentioned about his weakness and old age by referring to his bones. One of the reasons for this was, as pointed out in the translation, that in his peculiar circumstances i.e. his old age and physical weakness, it was not appropriate for him to ask for a child. Another reason given by Imam Qurtubi in his commentary is, that for possible acceptance of supplication it is better to mention one's weakness, destitution and inadequacy. For this reason religious scholars have emphasized that one should recount Allah Ta` ala's blessings and his own exigency before making his supplication.