Babies start to laugh at about four months of age.
Laughter in infants occurs about a month after smiling. It may be observed as early as 5-9 weeks of age and is firmly in place by 4 months of age. (Rose, Kenneth Jon. The Body in Time. p 150-152. NY: John Wiley & Sons, Inc., 1988.)
By about age 12 months a child begins to laugh at unusual or inappropriate adult behavior (e.g., funny faces, walking on all fours). By age two they begin to create their own juxtapositions and laugh at incongruities.(Branson, Roy, PhD. The Sacredness of Laughter. Vol 26, No. 4, January. p 45-46. WA: Spectrum, 1998.)
Children laugh more frequently each day than adults. Some reports indicate that children laugh an average of 300 times per day, while adults get in less than 20. (Sprague-Smith, Marilyn, M.Ed. C’mon, Let’s Laugh. The North Carolina Journal for Women.)
Boys and girls generally understand and appreciate humor equally. Boys, however, tend to enjoy more hostile humor (e.g., silly rhymes, naughty words, teasing). (Tanenbaum, Joe. Male & Female Realities, Understanding the Opposite Sex. p 152-154. NV: Robert Erdmann Publishing, 1990.)