Having your Will give a power of maintenance to your trustees means that your trustee is able to pay and release any part or the whole of the income of the presumptive share of any minor beneficiary for the minor's maintenance, education, benefit or advancement in life. This means the minor is not going to end up with no money to live on.
Giving the trustees a power of advancement means that your trustee is able to pay and release any part or the whole of the capital of the presumptive share of any minor beneficiary for the same reasons.
In simple terms if your Will gives a power of maintenance and advancement to your trustees the share of your minor beneficiary is not given out to him or her at 21 years of age. Then when he or she meets the condition you specify e.g. reach 25 years of age the share can then be paid to him or her. Until he or she meets that condition the trustees are able using the power of maintenance and advancement to provide for him or her in the meantime.
Delay the vesting age for your infant beneficiary but at the same time give the trustees the powers of maintenance and advancement. It will ensure your infant beneficiary will not end up in poverty in the meantime whilst allowing for him or her to be mature enough to handle the inheritance when the specified age is reached.