Paolo Cesare Maldini (Italian pronunciation: ['pa:olo mal'di:ni]; born 26 June 1968) is an Italian former professional footballer who played as a left-back and central defender for A.C. Milan and the Italy national team. He spent all 25 seasons of his career in the Serie A with Milan, before retiring at the age of 41 in 2009. He won 23 trophies with Milan while in the squad: the UEFA Champions League five times, seven Serie A titles, one Coppa Italia, five Supercoppa Italiana titles, four European Super Cups, two Intercontinental Cups and one FIFA Club World Cup. Maldini made his debut for Italy in 1988, enjoying a 14-year career before retiring in 2002 with 126 caps, a record at the time, which has since only been topped by Fabio Cannavaro in 2009 and Gianluigi Buffon in 2013.

On 16 February 2008, in the match against Parma at the Stadio Ennio Tardini, Maldini came on for defender Marek Jankulovski to make his 1,000th professional career appearance, of which 861 were with Milan, 12 with the Italy under-21 side, 1 with the Italian Olympic team and 126 with the Italy senior team. In European football, only the English goalkeeper Peter Shilton had collected more appearances at the time: 1,390 between 1966 and 1997. Maldini played his last game in the Champions League on 4 March 2008, a 2-0 loss at the San Siro in the last 16 against Arsenal which eliminated Milan from the 2007-08 Champions League. In 2008, Maldini was awarded the FIFA Order of Merit, as well as the Premio Internazionale Giacinto Facchetti, which is awarded to a player who was demonstrated both skill and fair play throughout his career.  On 18 April 2009, Maldini announced that he would be retiring at the end of the 2008-09 season. On 17 May 2009, in the Stadio Friuli, Maldini played his 900th official match for Milan in a league game against Udinese. Maldini's last match in San Siro was on 24 May, a 3-2 loss against Roma, and was given a standing ovation by the fans. There was a small controversy, however, when the Milan Ultras fans known as Brigate Rossonere protested against Maldini as he said goodbye. His last appearance for Milan, and his last game as an active player, was a 2-0 win against Fiorentina on 31 May 2009 in the last match of the Series A season. This win meant Milan finished in third place and qualified for the following season's Champions League. Maldini was once again given a standing ovation by the fans. As they had previously done with Franco Baresi's number 6 shirt, Milan retired Maldini's number 3 shirt, but stated that it will be bequeathed to one of his sons if one of them were to make the club's senior team.  On 28 August 2009 in Monte Carlo, Paolo Maldini was Awarded a prize for his career by UEFA during the draw for the group stage of the 2009-10 Champions League. On 17 November 2009, Spanish sports newspaper Marca awarded Maldini the "Marca Leyenda" prize for his career and achievements.

Using a quote from the above article, answer the following question: when did he retire?
On 18 April 2009, Maldini announced that he would be retiring at the end of the 2008-09 season.