My teenager made 3500 last year, do they have to file?

If he has a W-2 that shows tax withheld in boxes 2 or 17 he can file to seek a refund.   If no tax was withheld he does not need to file.   You do not include his W-2 on your own tax return.   You can still claim him as a dependent.   Make sure if he files a tax return that he says on his own return that he can be claimed as someone else's dependent.

If the money he earned was reported on a 1099NEC---he is required to file and will have to pay self-employment tax for Social Security and Medicare.  You might want to use the IRS Free File site in that situation so that you do not have to pay for the expensive self-employment software here.

https://www.irs.gov/filing/free-file-do-your-federal-taxes-for-free

No, your teenager does not have to file a tax return because their earned income, is less than $12,550.  Even if they do not meet the income filing requirements they may want to file a tax return if federal and or state income tax was withheld.