By Sara Webb and Anthony Deutsch

AMSTERDAM (Reuters) - Dutch Queen Beatrix, who turns 75 on Thursday, announced she was abdicating in favor of her son, Prince Willem-Alexander, who will become king on April 30.

Willem-Alexander, 45, is married to Princess Maxima Zorrigueta and has three young children. Decades of grooming for the throne involved shaking off his image as a beer-drinking fraternity boy whose blunt comments upset the press and politicians.

Beatrix said in a television broadcast to the nation that she was stepping down because she felt her son was ready to take her place on the throne.

A constitutional monarchy, the Netherlands had reduced the involvement of the Royal House in politics, a role long seen more as a formality than a position of power.

In the past, the Queen took part in forming government coalitions by appointing a political mediator, raising questions about behind-the-scenes influence on the democratic process.

That role was scrapped before the last election, which took place in September 2012.

It was widely rumored that Queen Beatrix was no fan of anti-immigrant, euroskeptic politician Geert Wilders. She alluded in speeches to the need for tolerance and multi-culturalism, comments that were seen as criticisms of Wilders' anti-Islamic views.

Wilders' poor showing at the last election and loss of influence in politics, could well have contributed to her decision to abdicate.

Queen Beatrix, who remains very popular with the Dutch, became the sixth monarch of the House of Orange in 1980 following the abdication of her mother, Queen Juliana, who reigned for 31 years.

Juliana was 73 years old and in deteriorating mental health when she abdicated but Beatrix has remained active and in good health despite some setbacks.

The queen was emotionally shaken when a man drove his car into a Queen's Day procession in 2009.

Her middle son, Prince Johan Friso, has been in a coma ever since he was buried in an avalanche while skiing last year.

(Reporting by Sara Webb and Anthony Deutsch; Editing by Robin Pomeroy and Giles Elgood)