Formidable form calculation custom code

I have a Formidable Form that I need help with. I'm stuck on one piece of custom code in particular, but I'm open to you helping with the entire form. This is for a tax prep firm. I need to take the tax year selected from a drop-down. Based on the year selected, calculate the person's age by the end of that tax year. Example: This year is 2020, but people are doing their taxes for previous years. For this example, let's say someone is doing their 2018 tax return. I would need to calculate ages by the end of 2018, not their age today. So, if Susie is 17 by 12/31/2018 a field won't show but if she is 18 by the 12/31/2018 a field will show. However, if the tax year is 2019. Susie is a year older. So if I'm doing John's taxes and Susie is his daughter and his tax dependent and Susie's date of birth is 01/11/2001. Susie would be 17 for John's 2018 tax return and John would get an extra $1,000 credit for Susie. However, for his 2019 tax return Susie would be 18 and John wouldn't get that $1,000 tax credit. So I need the age to based on age by the end of the selected tax year. I hope that makes sense. Thank you!