You've not stated under which legislation the charge is brought.
If it's under Section 43 of the Telecommunications Act 1984 (or under Section 1 of the Malicious Communications Act 1988), an adult could ace up to 6 months in prison and/or a fine of up to �5000.
If it's under Section 2 of the Protection from Harassment Act 1997, an adult could face up to 12 months imprisonment (and/or a fine).
If it's under Section 4 of the Protection from Harassment Act 1997, an adult could face up to 5 years imprisonment.
(See here for the differences between those pieces of legislation:
http://www.freebeagles.org/articles/malicious_ calls.html )
In practice, a juvenile who is appearing before a court for a first offence will be made the subject of a referral order:
http://www.yjb.gov.uk/en-gb/yjs/SentencesOrder sandAgreements/ReferralOrder/
A criminal record is for life but a referral order becomes 'spent' (for most purposes) as soon as the order ends. So the offender could lawfully answer 'No' to any questions from potential employers relating to whether he has any criminal convictions. (The only exception would be when applying for those types of employment which are exempted from the provisions of the Rehabilitation of Offenders Act, such as working with, or alongside children or vulnerable adults, or for jobs involved with the administration of justice).
Chris