Can you claim your child as a dependent? Then you are just paying medical expenses for dependents, it's not a gift and it would be a tax deductible medical expense on your tax return (subject to the usual rules). You could claim your child as a dependent if she is any age, as long as
1. she is unmarried, or she is married but she files her tax return as "married filing separately".
2. she does not have her own dependents to claim.
3. her gross taxable income is less than $5050 for 2025.
4. you provide more than half her total financial support.
If she is not your dependent, then you would have to file a gift tax return if your gifts to her are more than $19,000 for 2025. However, you can stretch the limits if you or she are married. For example, if you are married, then you and your spouse can each give her $19,000 before triggering the form. And if she is married, then you and your spouse could give a total of $76,000 (4 individual gifts of $19,000 in each pairwise combination).
