Del and Rodney's age-gap has been widely disputed.
It all depends on the stories they tell.
On their mum's grave, it says she died in 1962. Rodney was born in 1960 according to Rock and Chips prequel. Yet we hear Rodney was 4 or 5 when Joan died.
A more in-depth copy and paste job from Wiki:
The BBC comedy guide for Only Fools and Horses clearly states that Rodney's birthday is 26 February; but this is contradicted in "Sleepless in Peckham", when Cassandra mistakes Freddie the Frog in an old 1960s photo of the first Jolly Boys' Outing for Rodney. Rodney corrects her, claiming that the photo was taken in July 1960 before he was born (according to Rodney), thus making Rodney's date of birth the date he reveals it to be in "The Unlucky Winner Is...", 2 November. This is confirmed when Rodney's birth is shown in "Rock & Chips".
However some episodes of Only Fools and Horses do not support this; in "Big Brother", 1981, Rodney states to Del that he is 23 years of age, which would make his year of birth 1957 or 1958. In the same episode Del Boy states that there is a thirteen-year age gap between himself and Rodney. "You couldn't be like any other little brother could you eh, and come along a couple of years later after me. Oh no no, not you. You had to wait thirteen years!" In the following episode "Go West Young Man, which was broadcast in September 1981 (and was the second episode of Only Fools and Horses ever broadcast), Del Boy states that he (Del) is 35. In "Big Brother" since Rodney had said he was 23, Rodney must already have had his birthday that year with Del still to have his 36th birthday later that year. Therefore Rodney must have been born during 1958. In "As One Door Closes", set in early 1985, Rodney tells Del that he is 24, making his year of birth 1960. In "Yuppy Love", Del states that the Trotter family have been living at Nelson Mandela House since 1962, and that Rodney was born in the flat. Similarly, Rodney & Cassandra's wedding certificate shows that they were married on 25 January 1989 and gives Rodney's age as 26 and Cassandra's as 21. This would make Rodney born in 1962 (assuming he hadn't yet had a birthday before his wedding in 1989) and during "Modern Men" (set around Christmas 1996), Mickey Pearce describes Rodney as a "34-year-old paperboy", which would also back up a birth year of 1962. However, during "Mother Nature's Son", set around Christmas 1992, Rodney states that he is 31, suggesting he was born in 1961.