In "The Basement Room," is Philip too young to understand adults' problems?
Philip is a young boy who has been loved, admired and sheltered. He has never had unsheltered exposure to or experience of evil. He therefore still believes in the goodness of good. He identifies Baines as good while he identifies Mrs. Baines as evil and that which is fearsome. He has been sheltered from her by the presence of his parents and the distance from her that they afford him.
When his loving and well-intended parents take their ill-fated trip away from home, Philip is thrown in close proximity with the mix of good and evil that Baines and Mrs. Baines represent. The author's thesis is that with maturity comes the ability to accept the responsibility and burden for another person that accompanies love for another and that Philip had not reached an age, a level of maturity, that prepared him with a psychology and an emotional range deep enough and broad enough to be able to uphold and fulfill the burden and responsibility of love.
This thesis is dramatized by the fact that the three instances in which Philip was burdened by the responsibility of love were of great import. So yes, based on the story, it is Greene's thesis that Philip was too young--and in being young, too unprepared--to understand and bear up under an adult's world.