Is istikhaarah valid on the part of a boy who has not yet reached puberty?
Question

I would like to ask you about Istikarah that is there any age limit for Istikarah mean someone has to be an adult for doing istikarah? And also would like to know that is there any harmful affects after doing Istikarah?
I will be grateful if you answer my those questions.
Praise be to Allah.Istikhaarah means seeking guidance from Allah, may He be exalted, to choose the best option. What it involves is delegating the decision to Him, may He be glorified, for He knows best what is good for His slave and is able to bring about what is best for the one who is seeking His guidance, if he calls upon Him and asks Him to guide him to the best option.
Fayd al-Qadeer (5/442)
Istikhaarah prayer is a prayer offered by the individual when he is thinking of doing something and does not know whether it is better to do it or refrain, so he asks Allah to guide him to the best choice regarding it.
There is no particular age for offering istikhaarah prayer, so it is valid on the part of an adult or the part of boy who has not yet reached puberty, if he has reached the age of discernment.
Acts of worship are valid if done by a child who has reached the age of discernment. In most cases the age of discernment is around the age of seven years. Therefore the Prophet (blessings and peace of Allah be upon him) told us to instruct our children to pray when they are seven years old.
If obligatory prayer is valid on the part of a boy who has reached the age of discernment, and it is even valid for him to lead the prayer, as we have seen previously in fatwa no. 155061, then it is more appropriate to say that a supererogatory (naafil) prayer (including istikhaarah) offered by him is valid.
The Muslim should combine asking Allah, may He be exalted, to guide him to what is best with consulting people who are religiously committed and wise, because as the saying goes: He will not be disappointed, whoever asks Allah to guide him to the best option (istikhaarah) and he will never regret it, the one who seeks the advice of others.
Istikhaarah is all good for the individual, and there cannot be any negative effects of istikhaarah, because it is praying to Allah, may He be exalted, calling upon Him and delegating the matter to Him – and all of that is good, and nothing bad can result from what is good.
So the Muslim should pray istikhaarah and consult others, then put his trust in Allah and make a decision, and Allah will decree for him what is good, in sha Allah.
And Allah knows best.