Dependent minor child is up to 18 but when an adult has no spouse and financially supports a son/daughter or parent, they can claim the eligible dependent credit if the financially-dependent's income is less than the basic personal credit (a bit over $13k).
If you have a spouse with little/no income (again, below basic personal exemption), you get the same credit. The eligible dependent simply gives the same treatment to unmarried people similarly supporting another adult financially. They used to call it "equivalent to spouse" credit. But it absolutely does apply to this situation.
https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-return/completing-a-tax-return/deductions-credits-expenses/line-30400-amount-eligible-dependant/you-claim-amount-eligible-dependant.html
Hey UFILE, why is your software not functioning properly for this credit?