What is a Raga ? To put it in simple terms, a combination of swara-s in certain order of ascent called arohana and descent called avarohana. Essentially it must have some aesthetically melodic content. AlApana is a means to communicate to the audience, the flavor or the bhAva of the rAga through the permitted notes and its phrases.An alapana, is the exposition of a raaga or tone – a slow improvisation with no rhythm. In presenting a phrase of Alapana, the swaras are not spell out as such, but spell out as certain syllables like tha dha ri na.

Well, pointing out an age range might be unfair here yet I would say ‘from childhood’.

Carnatic music is an ocean. The joy you get from learning is enormous. :)

Moreover , things that you start learning early in life would stay close to your heart and intact in your mind .

I have  students from age 4 to 55 both online as well as face to face learning ones. There is a formal system with lessons that can be taught in the sequence - saraliswaras, janta swaras, alankarams(talams), geetham, varna, krithi and divya naama sankirthams along with  light items, Bhajans etc.

 Who told it is not as popular as Hindustani. Infact carnatic music is taught across the globe even in UK and US. U have carnatic thyagaraja festival conducted in Melbourne, Cleveland and nuke and corner of the world. People south of India just after Bombay related only carnatic music as classical music. Infact carnatic music has as much base, theory and strong root as Hindustani.

Yes , there are many apps, vidoes available online to learn Carnatic music by oneself. But it is always recommended to learn it from a guru/teacher it is the key of learning good music is listening to masters. Get a good internet plan and start listening to renowned maestros online. Just follow how they render ragas and swaras.
And practice a lot.