How old was Joseph when he was sold into slavery by his brethren?
The Bible explicitly states in Genesis 37:2 that Joseph was 17 years old when he shared his dream with his brothers, who “ hated him yet the more ” ( Genesis 37:5 ).
It didn’t take long for an occasion to arrive for his brethren to manifest their envious hearts ( Genesis 37:11 ) and rid him by selling him to the Ishmeelites/Midianites ( Genesis 37:27-28 ).
It may be argued that Joseph was 17 years old when he had his dreams , but was older when he was taken into slavery. However, a point to emphasise is Reuben, when coming back to the pit and to his brethren states:
And he returned unto his brethren, and said, The child [is] not; and I, whither shall I go?
Genesis 37:30
Reuben also repeats this again when he chides his brothers in Egypt after Joseph has kept them in prison for 3 days:
And Reuben answered them, saying, Spake I not unto you, saying, Do not sin against the child ; and ye would not hear? therefore, behold, also his blood is required.
Genesis 42:22
Reuben deems Joseph a child (requiring dependents) and this is why Reuben wanted to deliver Joseph out of the of his brethren ( Genesis 37:21 ).
This is also interesting as it shows the maximum age of a child in Scripture to be at 17-18 years of age!