I thought Reg left Del and his family on Del's 16th birthday??

They've had to change a lot for Rock N Chips. Originally Del stated (in the pilot episode of OFAH) he was 13 when Rodney was born and his Mum was 39 and mistook the pregnancy for the change. In the very early episodes of OFAH's Joan was portrayed more as an old cockney duck who wasn't in good health in her last few years. The glamour and reputation only built up with further episodes and series.

When Reg makes an appearance in the show it transpires he left Del to fend for himself with Rodney (who was 5 or 6) right after Joan died.

By the time we get into the late 1980's and 1990's (and the Christmas episodes), more dates are becoming clearly defined as to when Joan was alive and how old Del would have been when Reg left. It was always felt he was around 17 when Joan died and Reg left, making his year of birth 1947 which was in keeping with the age Del was supposed to be when the series started in 1981.

It was only in Rock N Chips they made him 16 in 1960 which would have meant his year of birth was 1944 and this effectively added 3 years from all the original dates given in OFAH.