Callistemon are Autrailian plants and I am not completly sure but if no one knows the answer on here you can always email or ring up the RHS and they will answer questions on anything plant based.
I am not sure if you need to be a member but I dont think so
They have boffins lol at Wisley in the herbarium, who constantly look up things for the puiblic. They take it very seriously but dont always answer immediatly but when they do it is really comprehensive
They will contact specilists and people who have contacts if they dont know the answer immediately.
They probably will know the answer to that as a specialist in them will know.
I do know that they can vary alot from seed and you should get a really good mix from seed.
If you want to get the same plant then you need to take cutting instead.
I am a heuchera specialist sorry.

Some seeds as long as they dont get damp and kept in the right conditions will germinate for years.
Heuchera (my speciality) for instance I would still sow 2 years from collecting maybe even longer with some varieties.
Like wise others need to be as fresh as a daisy.
Some seeds in Autraila for instance need a bush fire to stimulate them to grow. These seeds can lay in the ground for 10's of years and more waiting for the right conditions
Other seeds need to be really cold to germinate.
Always a good idea to look where the plants orignate from and what that plant does in the wild because that will be the best way to imiatate the wild.
example:-
Many perennial native to this country set seed during the summer and drop their seeds on the ground and need some colder weather to help them and then come up as seedlings the next spring.
Others like the weed ragwort which needs pulling if you see it as it kills cattle can lay in the ground for 100 years (ragwort is a tallish weed with mops of fluffy yellow flowers, tends to grow in farmers fields and grass verges)
Hope that helps.

This link might help..suggests older seed is better, and using the 'bog' method.

http://asgap.org.au/callis2.html