Kids often find it awkward when a parent begins dating, no matter what age they are. It doesn’t matter if they became single again through a breakup or through a bereavement; it can take a significant amount of time to adjust to a mom or dad seeing somebody new, who may well end up becoming a big part of family life in the future.
While it is extra important as a parent to introduce a new partner slowly while children are still children, it is also often a good idea to do the same thing when they are adults. It can take a long time to adjust to the idea of seeing a parent with somebody new. Even if it seems like things are going well, they are not necessarily going to be open to including them as a part of the family straight away. These things always take time.