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Actually, IF he is considered permanently and totally disabled, then his age does not matter.  The KEY is whether or not he is permanently and totally disabled. If he is, then he is considered to be a qualifying child and as long as he did not provide more than half of his financial support, then you can still claim him.

If he IS NOT totally and permanently disabled, then his investment income of $8.600 would disqualify him from being claimed as a dependent on your return.

Qualifying Child does not mean he qualifies for the Child Tax Credit as children no longer qualify for that once they turn 17, instead they qualify for the Other Dependent Credit but are still considered Qualifying Children as long as they meet the criteria to be treated as so.  

If you walk through the personal info questions on TurboTax, and select that he is  disabled (IF HE IS), then you will see the question asking whether or not he provided over half of his own support INSTEAD of the question regarding HOW much he makes.  

So the primary considerations is IF he is permanently and totally disabled, regardless of age, and did NOT use that money to provide over half of his own support, he IS still able to be claimed as a dependent.

IF he is NOT permanently and totally disabled, then his unearned income IS too high for him to be claimed as a dependent. 

Qualifying child

To qualify as a dependent, a child must also pass these tests:

• Relationship: Be your son, daughter, stepchild, eligible foster child, brother, sister, half-sister or -brother, stepbrother, stepsister, adopted child or the child of one of these
• Age: Be under age 19 or under 24 if a full-time student, or any age if permanently and totally disabled
• Residency: Live with you for more than half the year, with some exceptions
• Support: Get more than half their financial support from you
• Joint return: Not file as married filing jointly unless only to claim a refund of taxes paid or withheld

See the full rules for a qualifying child

Qualifying relative

A qualifying relative must meet general rules for dependents and pass these tests:

• Not a qualifying child: Isn't your qualifying child or the qualifying child of any other taxpayer
• Member of household or relationship: Lives with you all year as a member of your household or is a specific type of relative
• Gross income: Has gross income under $5,050
• Support: Gets more than half their financial support from you

See the full rules for a qualifying relative

Permanent and total disability

A person has a permanent and total disability if both of the following apply:

• They can’t engage in any substantial gainful activity because of a physical or mental condition and
• A doctor determines their condition:
  • Has lasted continuously for at least a year or
  • Will last continuously for at least a year or
  • Can lead to death