The age gap between Nymphadora Tonks and Remus Lupin is three years.
Tonks was born in 1973, while Lupin was born in 1972. This information is confirmed in the Harry Potter books and other official sources.
While the age gap might seem small in the grand scheme of things, it was a point of contention for the couple, particularly for Lupin, who felt he was too old for Tonks.
It is important to note that the age gap was not the primary issue in their relationship. Lupin's fear of passing on his werewolf curse to Tonks and their differing views on starting a family were more significant factors in their struggles.
Despite their challenges, Tonks and Lupin were deeply in love and eventually married. They even had a son, Teddy Lupin, who was born in 1998.