A life sentence does not mean life imprisonment. that is two different things. Life sentence means that the person will be on license for life, so will be checked and moniotored, and if they break the conditions of that license they can be taken back into prison immediately, without a further court case for example.
Also when determining his sentence the court would have had to take into account he was a minor/child when he committed the crime, therefore the imprisonment term would generally be lower for various reasons.
Partly it would be that there is more chance of a child changing and becoming a more responsible and safer bet on release than if he had been an adult at the time of the crime, as people change a vast amount between the ages of 15 and 26.
Whilst is does seem very unfair to the memory of Mr Lawrence and to his family, the parole board will consider many factors before releasing this man. It is in not an autimatic right, and just because he is elgible for release does not necessarily mean he will be granted it straight away. Much will depend on how he had been in prison and whether the parole board believe him to have undergone significant rehabilitation. They will also set appropriate license conditions at the time, so he will not simply be freed to live life freely and normally.