As a follow up, how much of the victory would have been attributed to his abilities or would his troops have been under the command of a loyal general/commander?

Not to completely jump the rails here, but there are several non-European examples of young teenagers commanding armies and winning battles in the pre-modern period. One such example which leaps to mind is Shal Ismail, founder of the Safavid Persians. His father was killed in a war with a Turkic tribe and went into hiding at around 7 or 8 years old. He later emerged at the ripe old age of 12 to command a cult-like Shi'a Twelver sect called the Kizilbash - Ismail claimed to be descended from the blood Ali, the son-in-law and nephew of the Prophet Muhammad - which eventually swelled in number to some 7000+ a mere decade later. Ismail was seen as a kind of holy figure, reportedly never losing a battle since God (Allah) was on his side. Eventually he went on to conquer Iran, parts of Afghanistan, a good chunk of Iraq, and even chunks of Anatolia. Technically, though, he was a twenty-something when he hit his stride, but he was reported to have waged battles as a young teen.

Sources: A Concise History of the Middle East by Arthur Goldschmidt Safavid Persia by Charles Melville

We don't know when Olav was born. The year 995 is the year when Olav Trygvasson took over power in Normay. It is possible that the sage poets saw that year as fitting and chose it for that reason.

What you call a battle should better be called a viking raid. It is not clear whether Olav led the raid, but it is most likely that he stood out in the fighting.

So, he could have been born earlier or not, we don't know, as the sources are not considered the best.

Follow up question: I have heard that with young hereditary leaders like this the population didn't often know exactly how old they were. is this based on historical fact or just made up somewhere? How could the population not know if their birth was (likely) announced?

This thread should have some answers for your first question.

In 1645, Shivaji, a Hindu king began carving out a kingdom for himself from a rival kingdom (Adil Shahi Sultanate) at the age of 15 with the capture of one fort.

Over the next 34 years, he fought against the Adil Shahi and the Mughal Empire to extend his kingdom. He was crowned Chhatrapati or Monarch in 1674. This kingdom was the genesis of the Maratha Empire which, in 1760, controlled most of present day India and Pakistan.

I'm not a historian. Since I'm not sure if local folklore is an acceptable source, I won't attempt to go into details of his first few conquests. But I just wanted to point out that there were people who began building kingdoms in their early teens and succeeded.