The average age of the top women tennis professionals has taken a dramatic plunge in recent years, even as their average earnings have soared.

This simple fact of tennis life is readily apparent at the Colgate Series Championships, which come to a close with the finals tonight at the Capital Centre.

For example, the average age of the Colgate finalists in 1977 was 27 1/2 years, according to tournament promoter Edythe A. McGoldrick. This year, she notes, the average age of the finalists is 21 1/2 years old, with half of the eight singles players 18 years old or younger.

Tonight's matches will be the culmination of a nine-month tennis tour, during which the women gather points in a complex scoring system. Colgate put up $825,000 in tournament money, and the biggest point scorers ended up with a crack at $250,000 in prize money during this five-day period.

From here, the women move on next week to the winter tour, with Avon Products Inc. -- a competitor of Colgate in a number of product lines -- putting up the prize money.

The money earned by these young women is truly extraordinary. Raymond S. Benton, a Washington attorney whose firm, ProServ Inc., manages the affairs of Tracy Austin, among others, says that the top two females players, Martina Navratilova and Austin, earn as much prize money as the two top male players, John McEnroe and Bjorn Borg.

Going into this tournament, Navratilova had won $763,487 in prize money, plus bonus money from the $250,000 Colgate bonus pool of $34,000. Austin, who is just 18 years old, has earned $590,378, plus a bonus of $52,000.

But Benton adds that once beyond the top two, the prize money is far greater among male players than it is among females. The men also collect far more money from exhibitions and endorsements.

After Austin, the other top prize-winners were Chris Evert Lloyd ($448,553), Billie Jean King ($296,713), Wendy Turnbull ($308,413), Hana Mandlikova ($368,642), Evonne Goolagong Cawley ($218,505), and 15-year-old Andrea Jaeger ($220,496).