The right to have an offence stepped down would only apply to previous convictions that are no longer sexual offences. If, for example, you have been give a formal police caution or have a criminal conviction for an offence such as having sex in a public toilet, this is likely to be disclosed following a CRB check. A penalty notice (£80 fine) or a verbal warning given on the street would not normally be disclosed by the police.
Having a conviction for an offence such as cottaging doesn’t give an employer (or similar) the right to exclude you from working with any particular group, such as young people. If you experience problems relating to employment or discrimination you should consider getting legal advice.