I am working on three separate works for three different audiences. All are non-fiction. One is for children ages eight to twelve, another is for teens ages fourteen to twenty, and the third is for adults. Is it recommended that different names be used, or should they be published under one name?
They are all non-fiction, but what are the topics they are covering? If they are all something like, history books or biographies, there is no harm in using the same name for all three.
Non-fiction doesn't seem to have the same issues with crossing genera or age ranges that fiction does. So unless there is a pressing reason to try and keep them disconnected, I would just write them all under the same name.
Maintaining three pen names means three different brands, with three different marketing efforts. It takes a lot of work.
Before you decide, you would want to consider the following:
Will any of your audiences react negatively if they find out you also write for the other audiences?
For example, will some people devalue your business advice if they know you also give dating advice? Will certain people reading your more serious books take you less seriously if they know you write children's books?
If the genres do not conflict with each other, then there is less need to have different pen names. But if they do, then different pen names may be preferable.
You may also want to consider:
Will having a separate brand increase sales and make it easier to market?
Taco Bell, Long John Silver's, KFC and A&W restaurants are all owned by the same company, yet have different brands, because it makes them easier to market.
If you have a specific pen name (which is a brand) for one of your audiences, will that make it easier to sell to that market? Sometimes a more focused brand is easier to market than one that represents everything, or multiple markets/audiences.
For example, are you more willing to buy dating advice books and courses from David DeAngelo or Eben Pagan? Both are the same guy, but David DeAngelo is a better pen name for marketing dating advice.
Will you feel more freedom to write if you use a different pen name?
Sometimes using a different pen name allows you to use a different voice than you otherwise would, or express certain things you wouldn't normally say under your real name or main pen name. If this is the case, a different pen name may be advantageous.
Are you willing to maintain multiple brands?
It does take a lot of extra work, but in some cases, it may be better to do so. Before you decide to use multiple pen names, consider the added workload it may create.
In the end, it is up to you. Good luck!