What is best age range to train baby to sleep by her/himself? Anyone has recommendation?

Start as early as possible, and get into a routine and stick to it everyday. Your baby will soon recognize the signs and know that its time to sleep/nap. For me, this is how my schedule is like for my son. He wakes up at about 6 - 7am in the morning, plays for about 2,3 hours before putting him in his bouncer for nap. Nap varies from 15 to 30mons. He plays for an hour while my mother prepare his meal, and approz. 1.5 it'd bath time and nap again in his bouncer. Wake up and a short nap 2 hours later, and he will be up at the way until I come home. I'll play with him for a little while, then I'll wipe him down and change him to his PJs. Massage him and by now he knows it's bedtime. I carry him to our bed (we co-sleep), let him lie on his side, pat his bum for a few minutes and he's gone for the night until tomorrow 6 to 7am.

I wouldn't call it training. For my child, it's more of establishing a routine which she can fall into. Her evening bath is around 6pm and her last feed is around 10pm where she is coaxed to sleep. She used to wake up for night feeds, but can now sleep through the night and only wakes up at about 7am. Most babies can soothe themselves to sleep. My baby shakes her head from side to side when lying down. It looks really strange but she dozes off soon after she does that. She also sucks on her thumb to sleep.

Why is there a need to sleep train? It's more of a routine. My 2nd son is able to sleep himself by lying down with us. We will just do our things whereas he will just doze off himself

As early as possible to prevent any difficulties. Babies are easiest to teach when they are n ewborns.

thanks