Mel,
Cautions under the age of 18 are usually not disclosed as they are part of your police record and not part of your criminal record.
At 18 the criteria to receive an adult caution would not consider the previous junior cautions ( correctly called Repremand's and Final Warning ).
If you apply for a position of trust ie Police, Court working job with money/ child care, nursing etc. - an enhanced disclose will usually show all previous convictions. Cautions, Repremands and Final Warning are not convictions and therefore will normally be discounted by employers. However to receive a Caution etc you need to state you committed the offence and the Police have enough evidence to charge you to court if you don't admit it.
I understand that these cautions will not longer to able to be removed unless further evidence appears that the original investigation was compromised.
Remember to get a caution you have to admit the offence - was it to be remove it would mean you lied about admiting the offence which could cause you more problems.
Difficult situation but CAUTIONING saves court time with minor offences as does Fixed Penalty Tickets.
GOOD LUCK