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

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.