The heating fluid contains an antifreeze that also protects against corrosion and bacterial growth, but these qualities deplete over time.
To make sure the fluid is in good condition and still giving protection to the system the glycol manufacturer's give a time limit.
For the G13 antifreeze this is given a 5 year life.
Most of the blue antifreeze is a 2 year life.
If you do not know what is in the system then assume it is 2 year.
In some German manufactured vehicles, you will find a yellow antifreeze. This can be a 3 year, however if in doubt use the 2 year rule.
If the glycol has been contaminated with too much tap water or any incompatible antifreeze, drain and change immediately.
Mix blue and pink only if 100% certain what is in there ie. you have owned the van since new - 10%/1 litre mix maximum.
Top up with maximum 10% water - again only if you are certain this has not previously been done/in an emergency.
Ph levels must remain neutral.