Yes, the sons of Fëanor attacked the Havens of Sirion in F.A 538 and it was then that she cast herself into the sea - but not before leaving Aranrúth to Elros. Elros was born in F.A. 532, so he would have been 6 years old at the time. What happened to Aranrúth during their capture? I'm not certain.. I don't think Maedrhos ever makes any mention of it. You're right, it is a curious point.