My Ex mailed me a check for 3 years worth of child support. What are the legal ramifications if I cash the check?

Hi First, if you reside in NYS, the emancipation age is 21.  He cannot stop paying at 18. Second, you can accept the check, write "under protest" on it and cash it.  You can let him know that you are accepting the check, but that he must continue to pay you either from his disability payments or otherwise. Third, if he is considered disabled, and he is collecting disability through Social Security, you can apply under his Social Security number to receive support payments for your kids.  That would end when they turn 18 though.   I'm assuming you have an order requiring your ex to pay child support.   I would accept the monies but with the condition that this is nowhere near what he will be required to pay for the kids until they are each 21. No, child support is not a taxable event.  You do not pay tax on child support. I hope this helps.