My 7 month old has now learnt how to sit by themselves and has made one successful attempt at standing on their own by holding onto the couch. I know his cot mattress height is meant to be lowered when babies are older but I can’t find any information on when it should be done and the safety issues with this. Thanks.

The following is recommended from Product Safety Australia (ACCC) to reduce the risk of baby falling from the cot.
“Safety at home.

Set the mattress base to the lowest position before your baby can sit up.”
https://www.productsafety.gov.au/system/files/Safety alert — Cots.pdf

Some safety organizations would say once baby is pulling up on side of cot, but this is mostly related to portable cots, especially if baby is bouncing while holding the sides.
Regardless, I think it is certainly time to put the cot mattress to lower level.