Stronger sentences only affect things AFTER the criminal act.
A person who is going to shoot a 14 year old boy is hardly NOT going do it just because they may get a few extra years in prison.
Sadly much of this is down to immigration as many of the people involved are from ethnic groups and many are involved in drugs or other crimes.
Sadly it is too late to do much about it, the people are here and we have little control over them.
A couple of weeks ago a lady was driving a car in Brazil and by mistake drove in to one of the slum areas (favela).
Because she had a "nice" car (hire car) she was threatened and then shot when she did not hand over her car.
Sadly we are getting these sort of no-go areas in the UK. Areas full of mostly immigrants who have little in life and try to get what they can by any means, be that a stabbing, shooting etc.
News article about the Brazil crime here:
https://www.theguardian.com/world/2017/aug/07/british-woman-shot-after-family-stray-into-brazil-favela
Almost all of these problems in the UK are caused by our long time lack of control over immigration.
Sadly we are stuck with it.