My daughter, in her mid 20s, has been using one of my '99s as her daily for a few years. It's a Tiptronic nearing 150k miles. It's been a good reliable car for her and if I wasn't maintaining it, it would probably average about $1,000 a year for maintenance - tires, starter, and a few misc minor repairs.
There have been some others, but when I think of 16 - 18 year old 986 owners, the experience of these two comes to mind. Read some of their threads.
986 Forum - for Porsche Boxster & Cayman Owners - View Profile: BIGJake111
986 Forum - for Porsche Boxster & Cayman Owners - View Profile: cornontherob