Minors can invest in Mutual Funds through their parents/guardians. The minor is the first and sole account holder in this case and is represented either by a natural guardian (father/mother) or legal guardian (court appointed). A minor represented by natural guardian attains majority at 18 years of age while those represented by legal guardians attain majority at 21 years of age.
Once the Minor attains Majority, you need to apply for change of status of the sole account holder from Minor to Major or all future transactions (SIP/SWP/STP) would be suspended in the account otherwise. Usually Mutual Funds send a notice to the guardian and the minor for submitting necessary documents in advance. Guardian needs to apply for changing the status to Major along with Minor’s signature duly attested by a bank official. Bank account registration form and KYC of the Minor also needs to be submitted along with the application.
The tax implications will now have to borne by the sole account holder (Major). Until the child is a minor, all incomes and gains from the child's account is clubbed under the parent/guardian's income and the parent/guardian pays the applicable taxes. In the year the Minor attains Majority, he/she will be treated as a separate individual and will pay taxes for the number of months for which he/she is a Major in that year.