Apprentices and Boy Entrants in both the RAF and the Army undertook quite lengthy technical training and would not normally have finished their courses before they were 17 for Boy Entrants whilst Apprentices were usually 19 before they entered adult service.
In the late '70s, there were several 16 year old FLMs on the flight lines of UK and RAFG bases. They were generally no more likely to be casualties than any other RAF tradesmen working on the same bases. However, in both the young FLM case and for young teenage soldiers, they generally had still not finished growing and their physical strength and stamina was not as high as airmen and soldiers in their 20s.
Thus I would tend to agree that entry to any branch of the armed services where physical strength and stamina was a requirement for operations should not be allowed for under 18s. However, I would support the continued recruitment of Apprentices at 16 since they would not be exposed to active service until they were 19 and in most cases rely on skill and dexterity rather than brute force to meet their tasks.