The minimum age for most social media platforms is 13 years old. The use of 13 years old was put in place not to limit children’s exposure to inappropriate content but because of the Children's Online Privacy Protection Act (COPPA), which prevents companies from collecting certain information from under 13s. This being said I believe that 13 years old is a good minimum age when discussing access to the powerful possibilities of social media with children. WhatsApp recently changed the minimum age for use of their platform to 16 for users in Europe.
All social media platforms come with terms and conditions which are agreed to as part of the creation of a account. The Children’s Commissioner in the United Kingdom has produced simplified terms and conditions statements for Facebook, Snapchat, Instagram, WhatsApp and YouTube. These are well worth reading and discussing with your children, if are talking about the creation of social media accounts or if they already have them. The relevant webpage is here - please note that the WhatsApp resource does not reflect the recent change in minimum age to 16.
My advice is always to discuss social media platforms and accounts with your children, rather than just saying NO. A discussed, known about account is far easier to monitor and influence than one that is a secret!