My baby is 17 months old and I’m wondering what the safe age for babies to sleep with a blanket instead of a sleeping bag

Red Nose has the following information about soft/loose bedding (including loose blankets & doonas)
Older babies in a cot can be at an increased risk of a sleeping accident by using pillows and bumpers as a step to climb up and fall out of the cot. It is safer to wait until the child starts to sleep in a bed before introducing a pillow or other soft bedding.
https://rednose.org.au/article/at-what-age-can-i-introduce-cot-bumpers-and-pillows

Sleeping baby in a sleeping bag will prevent bedclothes covering the baby’s face.

If blankets are being used instead of a sleeping bag, it is recommended to place baby with his or her feet at the bottom of the cot, using layers of lightweight blankets that can be added or removed easily according to the room temperature. Tuck blankets in firmly so they cannot become loose and cover baby’s head during sleep.
https://rednose.org.au/article/how-much-bedding-does-baby-need

At 17mths of age I’m sure your baby is moving around the cot a lot.
Most babies who are mobile don’t seem to stay under blankets, which is why a sleeping bag is preferred.
Are you concerned your baby is cold?
If so, you can always add an extra layer of clothing under the sleeping bag.
if you are planning on moving baby from the cot soon, then it would be preferable to no longer use a sleep bag due to risk of falls.
file:///C:/Users/lorra/Downloads/RN3356_Cot_Bed_DL_Oct2018_Online (1).pdf