The cut-off for a dependent is 18, but if he is in high school, he continues to get benefits for the months he is in school up until he graduates or turns 19 years, 2 months old, whichever occurs first. I've two sons that collected benefits for several years, the younger one turns 18 in September but, after some paperwork where the school certifies he is a student, his benefits will continue until next June 2017 when he graduates. The older one collected benefits until he was 18 years, six months old but his benefits stopped when summer vacation started and he would have needed to re-apply for benefits when school started last Fall. We did not bother as the younger son's benefits doubled due to only one child (instead of two) was collecting to total to the calculated max family benefit. In other words, as a family we were collecting the same total benifit regardless if it was split between one or two dependents.
When the older child turned 18, the benefits were no longer sent to me as a parent/guardian , but had to be sent to a bank account that was in his name and SSN, not mine.
I'm pretty sure you would need to apply for SS benefits first, then request benefits for dependents. I believe you can not file and suspend unless you are at FRA, but I could be wrong. In my case, I started benefits at 63.5 years and am currently 66.
Somehow I thought you just recently retired!