This is kind of the basic facts of the case. Allen's relationship with Soon Yi was skeevy but not illegal.

You can look at any of the news reports from when this came out to find her age. Lots of the NY papers from back then are online.

I just did a search and here's an article on Woody and Mia from the year before the scandal broke. So this is not rationalization afterwards.

http://www.nytimes.com/1991/02/24/magazine/woody-and-mia-a-new-york-story.html?pagewanted=all

She and Previn had three sons (Matthew, Sascha and Fletcher); then they adopted three orphan girls -- two Vietnamese (Lark and Daisy) and one Korean (Soon Yi). After their divorce, she adopted a Korean infant (Moses Amadeus Farrow), who has cerebral palsy. In 1985, Allen and Farrow adopted a newborn girl from Texas (Dylan O'Sullivan Farrow). Two years later, they had their own son (Satchel O'Sullivan Farrow, named after Satchel Paige, the baseball pitcher and a hero of Allen's; the last name was chosen because the couple didn't want one Allen among two Farrows and six Previns).

..... Allen and Farrow very much lead their own lives, while continuing a relationship that is the longest either has had. By any standard, it is not a conventional union. They are not married, neither do they live together; their apartments face each other across Central Park.

If you search "woody soon yi 1992" here's the descriptions from the summaries you see on the front page of google

...discovered sexually graphic Polaroids of her 19-year-old adopted daughter...... ..... an affair between Mr. Allen and Soon-Yi Previn, Ms. Farrow's 21-year-old adoptive daughter...... the relationship between her adopted daughter and Mr. Allen came to light when she discovered nude pictures of Ms. Previn, who was then 21,....Mar 20, 1993 - Mr. Allen, 57 years old, said that when he embarked on a sexual relationship with Soon-Yi Farrow Previn, 22, who was adopted by Ms. Farrow and her former husband, Andre Previn,.......Mr. Allen's romance with Soon-Yi Previn, 21 years old, whom Ms. Farrow adopted when she was married to.......while he was becoming involved with her 21-year-old adopted daughter, Soon-Yi Farrow Previn. .....Mr. Allen begins an affair with Soon-Yi Farrow Previn, the 21-year-old adopted daughter of Ms.