ABH on my CRB

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When i was 14 yrs old I was given a caution for ABH.It was a fight over a boy(stupid I know).Iam now 17 and have been doing a childcare qualification over the last year. I have been working in placement and have become so I am told a valued member of the team and get on well with all the staff and children. Employment opportuntities have recently arisen and I was told that I would not be considered by the company, as I have ABH on my CRB. It seems they are happy to have me work there, just don't want to pay me! Is this discrimination on my part or are they well within their right to do this.Also, how long will this be on my criminal record, and will I ever find employment in this sector?