I'm pretty certain you could have applied with your family last August. Even though you were already 18 at that time, you first came when you were a minor and you are still dependent on your family and you haven't formed an independent life of your own (say, by marrying). I'm sure you can still apply on SET(O).
"Where a child who was given leave to enter or remain with a view to settlement under Paragraph 302 and Paragraph 319XA applies for settlement (or further leave to remain with a view to settlement) on the basis of his parents' or sponsors application/status, the fact that he has reached the age of 18 since being granted limited leave with a view to settlement is not a basis for refusing the application."
http://www.ukba.homeoffice.gov.uk/s...Is/idischapter8/section5a/annex-m?view=Binary Page 5
"To qualify under the Rules, at the time of application and decision, a child must:
* be unmarried, and not at any previous time have been married;
* not currently be in or have previously formed a relationship with another person (such as a common-law or homosexual relationship) which could be said to be the equivalent of being married, except for name and legal recognition;
* still be living with his parent(s) and any brothers and sisters who are living with their parent(s), (except where he is at boarding school as part of his full-time education);
* not be employed full-time or for a significant number of hours per week (although the obtaining of a "Saturday" job or a temporary job during his school holidays should not be counted against him); and
* be wholly or mainly dependent upon his parent(s), (or relative other than parents, if appropriate), for both his financial and emotional support."
Page 5 & 6