Varies according to chapter and recruiting world. Most recruit aspirants young.
Pretty much all of them pick up recruits as Teenagers.
A fleet based chapter will sometimes have specific worlds it recruits from, sometimes will just 'loot' the population of worlds it's fought near, depending on the chapter's character and whether it sticks to predictable 'patrol routes' through the galaxy. Either way, a ship from the fleet will collect potential recruits.
As aspirants, they get basic training on the way to the fleet, along with extensive psychic, psychological, genetic and physical tests to make sure they're up to scratch.
Once they get to the fleet, they're formally accepted, and become Neophytes (title may vary from chapter to chapter) - the process of implanting the assorted extra organs starts.
After that..... it depends on the chapter. 'Codex' chapters keep you in the scout company as you get used to your new abilities, and put you into battle in controlled experiences under the eye of a Veteran scout-sergeant (essentially drill sergeant). Once you're deemed good enough and there's a vacancy, you get your black carapace implant, a suit of power armour, and are attached to a squad in one of the 'proper' companies (normally a Devastator squad in the reserve companies).
Different chapters do the training bit in different ways. Black Templars assign a neophyte to a specific marine in a squad as a 'squire' and they learn on a one-to-one basis. A bit better for individual skill, a bit worse for tactical cohesion. The space wolves essentially go "screw training" and rush neophytes up to black carapace implantation as soon as their bodies can handle it, and throw them onto the battlefield half-trained (by
astartes standards - they're still at least as good as any 'normal' soldier) and let them figure out the whole 'battle' thing on their own from inside power armour.
#4 can you modify a fleet base chapters battle barge/ flagship? if you can how would you approach it?
Yes and no. Each chapter's battle barge/flagship is likely to be different. The idea of battle barges as a 'class' of ships in the Imperium is vague at best when each one is built on different forge worlds, centuries apart, and represents decades of effort such that very few people would live long enough to ever work on two of them.
So they will all be different (a bit). Modifying a ship is easy enough if the modification is small (one ship might carry less broadsides in return for more hangar space, for example), but the larger in scope the modification, the more likely it is you'd need a forge world's drydocks to do it; the chapter couldn't do it themselves.
