Sing Street is a brilliantly made film, and I definitely recommend it for the whole family.
The reason I think it is better for 15+ year olds is because of all of the smoking. There is a LOT of teen smoking, and at this age it is so easy to slip a cigarette into your mouth. The smoking isn't confronted at all in the film, and is evident from start to finish.
Another distressing factor in this movie is the swearing. There is one very clear, loud use of the word 'f--king' by a respectable character in the middle of the film, and plenty uses of 's--t' and 's---hole'. In one of the scenes, the word 'nigger' is visible in the background (if it is offensive to you).
However, it is my belief that it is better for children to know these words exist, rather than they be confused when they come up. It is still more suitable for older audiences, though.
There are two mentions of sexual intercourse, but not graphically. There is a very suggestive drawing shown close - up in one scene, which depicts of an penis emitting drips (possibly semen).
This title contains:
Drinking, Drugs & Smoking