Born and raised on the Portuguese island of Madeira, Ronaldo was diagnosed with a racing heart at age 15. He underwent an operation to treat his condition, and began his senior club career playing for Sporting CP, before signing with Manchester United at age 18 in 2003. After winning his first trophy, the FA Cup, during his first season in England, he helped United win three successive Premier League titles, a UEFA Champions League title, and a FIFA Club World Cup. By age 22, he had received Ballon d'Or and FIFA World Player of the Year nominations and at age 23, he won his first Ballon d'Or and FIFA World Player of the Year awards. In 2009, Ronaldo was the subject of the most expensive association football transfer when he moved from Manchester United to Real Madrid in a transfer worth 94 million ( 80 million).

Based on this passage, how old was ronaldo when he went to manchester united? Let's give stream of consciousness first:
To answer this question, we should know that: He underwent an operation to treat his condition, and began his senior club career playing for Sporting CP, before signing with Manchester United at age 18 in 2003.
So, the final answer is 18.