This idea has been used extensively to indicate the versatility of the Bard's work. Taking in to account Shakespeare's own lines of 'all the world's a stage', Shakespeare's plays in particular operate across social, cultural and age boundaries. Extracts of his plays and poems are experienced and enjoyed by even junior children (think of the captivating rhyme and ghoulish delight in the witches' spell in Macbeth) and yet appreciated on a deeper level by adults.
'All stages' refers to the versatility in terms of practical performance of the plays. Due to the intensity of description and the quality of characterisation, plays can be performed successfully in a range of stage configurations - proscenium, thrust or theatre in the round.
The Baconian theory here perhaps links to the questions surrounding the true authorship of Shakespeare's works. There is further detail around Bacon as an intellectual at teh e-notes link below.