The problem is that our cities' streets are become filled with litter. There has to be some kind of measure to stop it, and I believe the fines are a good idea in principle. It is also wrong to say 'well, he's only 13', because many kids that age will have been littering for years knowing full well that it was wrong. The other issue is, you can't really treat people differently, just if they look dodgy/chavvy. Would be obvious discrimination.
However, it seems that in this individual case the punishment doesn't really fit the crime, and there needs instead to be a system for repeat offenders and some kind of decent witness system. Also, the police do seem to have gone way over the top, as the litter dropped is certainly not significant and probably not even visible.
It's a fine line for the police to tread when making a decision such as this and it obviously needs to be made clearer.