Hi There, My boy turns 3 months in a few days, however, is adjusted age is just under 2 months. So just over 4 weeks early and he was a big boy at 3kgs, with no ongoing medical concerns. I am aware of the critical period of development, and the peak period of SIDS being between the ages of 2 - 4 months. When considering this do I use my boys adjusted age or birthday? Experiencing a touch of anxiety recently so have been sourcing some further information. Thank you

The critical development period is referred to as part of the research into Sudden Unexpected Death in Infancy (SUDI) which is SIDS & fatal Sleeping Accidents. This period is generally less than 6 months of age, but the majority of these deaths occur in the first 4-5 months.
https://rednose.org.au/article/why-are-safe-sleeping-recommendations-so-important

It is unusual for SUDI to occur where there are no modifiable risk factors (over 90% SUDI have risk factors identified in sleep environment.)
These risk factors include:
placing baby on tummy to sleep or baby not slept on flat surface.
head & face covered (eg loose bedding)
exposure to tobacco smoke
https://rednose.org.au/article/what-are-the-risk-factors-for-sudden-unexpected-death-in-infancy-sudi

Sounds like your baby is a well & healthy 3 month old.
Try not to be too fixed on specific ages, as these are predominantly guidelines & relate to averages of age.
Development milestones will occur at different rates for different babies.