Centers for Disease Control and Prevention (CDC) recommends all children to get two doses of MMR vaccine, starting with the first dose at 12 through 15 months of age, and the second dose at 4 through 6 years of age. Children can receive the second dose earlier as long as it is at least 28 days after the first dose. (1)
In the UK, the MMR vaccine schedule includes the first dose at one year old and the second dose at three years and  four months. If you miss any of these doses, you could still ask your GP surgery for them. (2)
In the USA, CDC recommends that any baby as young as six months old who will be travelling internationally should get a vaccine before leaving the US, followed by two additional doses later. (3)
Adolescents who haven’t been vaccinated should get two doses with 28 days in between. If they already had one vaccination, a second dose is required. (4)