(You talk as if this was carried out in the context of marriage...it wasn't)
Even within marriage...
Per http://www.wisegeek.com/how-has-the-average-age-at-marriage-changed-over-time.htm: If one looks at US statistics over the past 100 years for example, one sees that men had an average age at marriage of 25.9 years in 1900. Women in 1900 had an average age at marriage of 22 years. For some this shatters an illusion that women 100 years ago were sold into marriage as young children. Even Jane Austen, writing in the early 19th century had heroines married at the earliest age of 17 or 18. In Laura Ingalls Wilders books, which are semi-autobiographical, her father would not allow her to marry until she was 18. Thus it can be said that the average woman was past 21 when entering her first marriage, 100 years ago.
I am wondering how old the missionary was and what was the sex of the 16 year old.
Consensual sex, when there is an age difference of a mere 2 years, 18 months, or less, is not considered abuse. It could be that the missionary was 18 and the other teen was 16 and nearing the age of 17.
I need to know more before shouting “SEX ABUSE” for all to hear.