Hi there, my baby is 5months old tomorrow. She can roll back to tummy but not the other way. During the night she is rolling onto her side and falling asleep on her side. Should I be rolling her back onto her back? At what age can we leave them to sleep in whatever position they end up in? Thanks!

Most babies will be rolling over by 4- 5 months. ....some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side &/or tummy.
This is not something that needs to be stopped. It is important to make her new environment as safe as possible.

Are you still wrapping her? This should now cease, as she needs her arms free to roll safely.
Is she in a cot or a bassinet? She needs to move to a cot if not already.
What happens when you try to roll her back?

Ensure that the cot mattress is firm & flat & there is nothing soft or loose in his cot.
Always continue to place baby on back to sleep.
Increase tummy time when awake.

Red Nose has the following information now that your baby is starting to roll.
       
It’s essential to stop wrapping as soon as baby starts showing signs that they can begin to   roll, usually between 4-6 months. If you wrap baby, consider baby’s stage of development.  Leave arms free once the startle reflex disappears around 3 months.
If you are using a bassinette, it is time to transfer baby into a cot as soon as they first show         signs of being able to roll.

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over