There are times when an adult adoption is important, like if you need to take care of a relative with a disability or want to make sure someone you aren't related to is able to obtain trust funds without those funds going to your friends or family members first. The trouble is that some adult adoptions won't protect the person you care about in the case of your death, and they may not help him or her obtain the assets you wanted to leave behind.
Adoptions are incredibly difficult to reverse, which is one reason you should never go through an adult adoption without understanding the full implications. Once you adopt someone, he or she is under your care for better or worse. Even if the relationship ends, the individual is still likely to receive a portion of your estate.
In a trust, it may be the case that your son or daughter is next in line, not your spouse or partner. Maybe, for example, you never got married or don't want to get married. While an adult adoption could be a workaround, it's much harder to reverse than a marriage, and it may not be necessary. The law does require trustees to honor what you wanted to have happen, but even then, there could be issues that prevent the adopted party from obtaining assets.
If you think an adult adoption is the only option for you for whatever reason, it's advisable to speak with your attorney and to get to know your state laws. There are likely other ways to make sure assets go to the person you care about.
Source: The New York Times, "Adult Adoption a High-Stakes Means to an Inheritance," Deborah L. Jacobs, accessed Oct. 19, 2017