Is it legal for parents of missing teens to search SSN activity to try to find them?

The parents can't look up the SSN activity themselves. The parents can ask the police and/or the FBI to do so again if they aren't pursuing further any further search for the teen who is now an adult of the age of 21, which is the case as the case is closed. There are many children who go missing and are runaways and the police don't have the time nor the manpower to pursue cases of missing teens when the cases are old, and a case that is four years old is an old case, which is why they closed the case. It would be wise for the parents to ask the police or the FBI to run a Social Security search were the parents to know the Social Security number of the missing person. By doing that the parents may learn if the person is still alive as were there any recent Social Security activity the information will show up. Were there no activity it is possible the missing person may have died from anything including being killed by someone. Another way the parents may find the missing person and the last address is to file a form with the Internal Revenue Service (irs.gov) asking for a copy of last years tax return and stating their name as the name of the missing person because the IRS won't send the tax return to someone other than the person whose name is connected with the Social Security number. It is against the law for the parents to do so, but in an instance such as this and were the parents not to get cooperation from either the police or the FBI they may chose to break the law and do it anyway. Remember the police don't place missing children as a priority unless the child has very recently gone missing as that is the best time to look, and that is when an Amber Alert is placed. In most cases where an Amber Alert has been issued and the missing person isn't located within the first 24 hours there hasn't been much success locating them as many missing people have been found murdered after having been sexually abused. This is all too common when teenagers run away as there are predators out there looking for runaways to sexually abuse and kill.