Good question! I would like to know that too. Maybe the discussion will give us the answer.
I know that film commissions (who set the age to watch the films) in various countries think very different about it. In the U.K. for example Show me love is suitable (which means you are allowed to buy and watch it) for age 15 and over, but the advised age to watch is 18.
But what does it mean?
'15' – Suitable only for 15 years and over
No-one younger than 15 may see a ‘15’ film in a cinema. No-one younger than 15 may rent or buy a ‘15’ rated video or DVD.
Theme
No theme is prohibited, provided the treatment is appropriate to 15 year olds.
Language
There may be frequent use of strong language (eg 'fuck'). But the strongest terms (eg 'cunt') will be acceptable only where justified by the context. Continued aggressive use of the strongest language is unlikely to be acceptable.
Nudity
Nudity may be allowed in a sexual context but without strong detail. There are no constraints on nudity in a non-sexual or educational context.
Sex
Sexual activity may be portrayed but without strong detail. There may be strong verbal references to sexual behaviour.
Violence
Violence may be strong but may not dwell on the infliction of pain or injury. Scenes of sexual violence must be discreet and brief.
Imitable techniques
Dangerous techniques (eg combat, hanging, suicide and self-harming) should not dwell on imitable detail. Easily accessible weapons should not be glamorised.
Horror
Strong threat and menace are permitted. The strongest gory images are unlikely to be acceptable.
Drugs
Drug taking may be shown but the film as a whole must not promote or encourage drug misuse.
The self-harming thing is described here. I think this is one of the main reasons this movie has label 15 years and over in the U.K.
In the Netherlands for example there is no restriction, but only an advisory age to watch the movie, and that age is set to 12 years and over. First conclusion in this discussion might be: "It depends on which country you live"
But that's a quick conclusion just based on the movie certification boards.
I personally think this movie is harmless for every age group. But I recommend this movie as from age group 14 or so. Then the subject in the movie becomes really an issue. I know that in The Netherlands this movie is shown at school to children in that age group.
I will leave it at that for the time being, leaving this very important note:
You are certainly never too old to watch FÅ!!!