A lot of the articles on safe sleep refer to 'baby'. When it comes to safe sleep guidance, particularly for sleeping baby on their back, what age range is this guidance for? Also, which article provides the most up-to-date information about this?

The period of Infancy (or baby) is generally referred to as the first 12 months of life.
The majority of Red Nose research refers to reducing the risk of SUDI (Sudden Unexpected Death in Infancy) which includes SIDS & Fatal Sleeping Accidents.
SUDI is death of a baby less than 12 months of age where the cause of death is uncertain at the time.
https://rednose.org.au/article/what-is-sudden-infant-death-syndrome-sids
https://rednose.org.au/article/what-does-sudden-unexpected-death-in-infancy-sudi-mean

The first recommendation for Safe Sleeping is:
Always place baby on back for sleep from birth.
However, most babies will be rolling by 4-5mths & prefer to sleep on side or back.
Baby should still be placed on back, in a cot (not bassinet) with arms free (not wrapped/swaddled)
Once baby is able to roll well both ways they can be left in their position of comfort.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over