Note to Macbeth, Act 5, Scene 3, line 3.

How old was Malcolm?

Act 5, Scene 3, line 3.
Upon learning that his thanes are fleeing him and joining the forces led by Malcolm, Macbeth exclaims "What's the boy Malcolm?" Macbeth is using the word "boy" as an insult, but the insult would not have any sting if Malcolm weren't pretty young. However, when Malcolm appears he is not portrayed as a boy. Fleance is portrayed as a boy (See Act 2, Scene 1), and Young Macduff is portrayed as a boy (See Act 4, Scene 2), but not Malcolm, who in Act 4, Scene 3 conducts a very astute test of Macduff's trustworthiness.

So why does Macbeth refer to him as a "boy"? The only reason that I can think of is that perhaps that at the moment that Shakespeare wrote the line he had in mind the following paragraph from his primary source, Holinshed's Chronicles:
    The same night after, at supper, Banquho iested with him and said; "Now Mackbeth thou hast obteined those things which the two former sisters prophesied, there remaineth onelie for thée to purchase that which the third said should come to passe." Wherevpon Mackbeth reuoluing the thing in his mind, began euen then to deuise how he might atteine to the kingdome: but yet he thought with himselfe that he must tarie a time, which should aduance him thereto (by the diuine prouidence) as it had come to passe in his former preferment. But shortlie after it chanced that king Duncane, hauing two sonnes by his wife which was the daughter of Siward earle of Northumberland, he made the elder of them called Malcolme prince of Cumberland, as it were thereby to appoint him his successor in the kingdome, immediatlie after his deceasse. Mackbeth sore troubled herewith, for that he saw by this means his hope sore hindered (where, by the old lawes of the realme, the ordinance was, that if he that should succéed were not of able age to take the charge vpon himselfe, he that was next of bloud vunto him should be admitted) he began to take counsell how he might vsurpe the kingdome by force, hauing a iust quarell so to doo (as he tooke the matter) for that Duncane did what in him lay to defraud him of all maner of title and claime, which he might in time to come, pretend vnto the crowne.
The passage assumes that Malcolm is "not of able age to take the charge [of being a king] vpon himselfe."