Different offences have different rules although it is convictions, not records that become spent. Once "spent" they remain "on record".
Juvenile offences (under 18) remain on record for 3-5 years or until the juvenile turns 18 (whichever is the longest) after which they are removed, unless for setrious crimes or crimes of a sexual nature.
Adult convictions are considered "spent" but remain on record, hence the phrase "previous".