Is there anything from the books or movies that says how much older Petunia is than Lily? Or did J.K. Rowling just mention it in an interview?
Petunia is likely one to two years older than Lily (but possibly as much as five years older)
There's no specific birthdate given for Petunia but we can make inferences from the text and movie.
Lily is known to have been born in 1960
We know Petunia to be the older sister which places her birthdate prior to 1960.
There was undisguised greed in his thin face as he watched the younger of the two girls swinging higher and higher than her sister.
‘Lily, don’t do it!’ shrieked the elder of the two.
Harry Potter and the Deathly Hallows
- Petunia, eager to leave her parents' house went to London to do a typing course and find a job. The youngest age that it would be feasible for her to have lived alone is 18.
Harry's aunt and uncle met at work. Petunia Evans, forever embittered by the fact that her parents seemed to value her witch sister more than they valued her, left Cokeworth forever to pursue a typing course in London.
- We know from this deleted scene that she's been married to Vernon for a minimum of twenty years.
Petunia: I have lived in this house for twenty years.
Since this film was set in 1997, that places her marriage date at around 1977. Assuming she left home immediately that she was able, married Vernon the same year (at the age of 18-19) and moved into the family home immediately upon getting married, then her birthdate would have been approximately 1958-9, one to two years prior to Lily's.