Charitable giving is not limited to things you actually earned; just ask any volunteer who's donated time. If you want to earn tax credit for your gifts, you're not going to be able to do it with mileage gifts most of the time. That doesn't mean you should refrain from donating, especially if you're cash-poor and mileage-rich.
Donating miles is a good way to support charities without having to actually give them money, and some airlines contribute miles when you donate, netting the charity more total points. Donated miles help fly sick children and adults to places where they can receive medical treatment, or help soldiers fly home to reunited with loved ones.
While charitable donations of money or goods can be deducted from your taxes, charitable donations of miles cannot. The key difference: You own the things and the money you're giving away, but you don't actually own the air miles. Instead, they're a perk or reward you're getting, typically part of a loyalty program. Since you do not pay for the miles, you cannot legally deduct them. If you do try to deduct them, you may get audited. As the Red Cross notes, the IRS considers your miles a "gift" and not a charitable contribution. The IRS itself states that "money or property" are deductible contributions; these miles are neither.
If your goal is to get dollars for miles, you can gift them or sell them to another person. So help your child finance a vacation or pay for their ticket home with miles instead of cash to save money, or find someone who needs miles to buy them for cash. You can also just be charitable and donate the miles anyway. If your miles are about to expire and you know you won't use them, it's worth it.
If you actually paid for the airline miles and want to donate them, you can then claim it as a charitable deduction. In that case you can deduct the amount paid per mile, which typically average $0.02 to $0.25 per mile, notes SmarterTravel. You will need paperwork proving you purchased the miles in case the Internal Revenue Service challenges your deduction.