Daee comes from the Arabic root verb “da’aa” which means to CALL. Daee is a person who calls or invite. In Islamic terminology Da’eeis the one who calls people to belief in Allah (Glory be to Him), means inviting (others) towards the religion approved by Allah (Glory be to Him) i.e., ISLAM as mentioned in Qur'an.Surah Maidah5:3 “This day I have perfected for you your religion and completed My favour upon you and have approved for you Islam as religion.”
A good Da'ee usually requires knowledge in many aspects and religions. It is obligatory upon the Da’ee (caller) to have Ikhlaas (sincerity and purity of intentions and actions) for Allah - the Mighty and Majestic - neither desiring to show off, nor desiring reputation, nor desiring the praises and the accolades of the people. Rather, the Da’ee should only call to Allah seeking the pleasure of Allah (Glory be to Him).
The best person in the sight of Allah is a Daee who calls or invites people towards Allah as Allah said“And who is better in speech than one who invites to Allah and does righteousness and says, Indeed, I am of the Muslims." Qur'an.Surah Fusilat 41:33
There is not a specified age for a Daee. In Islam the deeds of a person gets recorded when he/she attends puberty. After this stage every Muslim should be a Daee, according to his level of authentic Islamic knowledge.
Dawah Guide (Towards performing Dawah) by Abdussalam