Do dragons of all sizes get lair actions and regional effects?

Question

In my campaign, a black dragon egg was lost by its mother and found by kobolds.

Now the wyrmling is about three years old, holed up in a mine, and being worshipped by the ever-subservient kobolds. Since this dragon is just a baby, I'm not sure if this mine constitutes a lair, or if it's powerful enough to exude lair effects.

My question is "Do dragons of all sizes get lair actions and regional effects?" but a thorough answer will hit all the following points:

• At what age is a dragon old enough to take lair actions while in its lair?
• At what age does a dragon's magic expand to the scale of Regional Effects?

Answer 1

Lair actions and regional effects are only permitted to legendary creatures.

From page 11 of the MM:

A Legendary Creature's lair

A legendary creature might have a section describing its lair and the special effects it can create while there, either by act of will or simply by being present. [...] This section only applies to legendary creatures that spend a great deal of time in their lairs and are most likely to be encountered there.

Lair Actions

If a legendary creature has lair actions, it can use them to harness the ambient magic in its lair.

Regional Effects

The mere presence of a legendary creature can have strange and wondrous effects on its environment, as noted in this section.

Since dragon's don't appear to gain their legendary status until they are adults (101+ years old) by reasoning that that's when they also gain legendary actions and resistance, only adult and ancient dragon's gain regional effects and can use lair actions.

This is further backed up, for regional effects at least, by the line:

Regional Effects

The region containing a legendary [insert colour] dragon's lair is warped by the dragon's magic, which creates one or more of the following effects...

under each dragon's description.

Answer 2

To start with, in case you're unaware: At 3 years old, your dragon is a wyrmling. Dragon age categories are provided in a table on page 86 of the Monster Manual.

Next, there's the question of which dragons have lair actions and regional effects. The sections describing each dragon type's lair actions and regional effects aren't attached to particular dragon sizes of each type, so it's possible that all dragons can have them. However, I believe the answer is actually adult or ancient dragons only, or 101+ year-old dragons.

Why do I think that? Well, on page 11 of the Monster Manual, it describes Legendary Creatures. It talks about their Legendary Actions, their Lair Actions, and their Regional Effects. The whole section is relevant here, so I won't bother quoting it, but the important bit is that:

Legendary creatures can take special actions outside of their turns

and

A legendary creature can take a certain number of special actions - called legendary actions - outside its turn.

I'm sure you can see where I'm going with this - only adult and ancient dragons have legendary actions, which leads me to believe that only adult and ancient dragons are legendary creatures, and therefore they're the ones with Lair Actions and Regional Effects. This is backed up by the fact that only adult and ancient dragons have Legendary Resistance. Also, the sections describing the Regional Effects of dragons all seem to use the following wording:

The region containing a legendary [colour] dragon is warped by the dragon's magic [...]

So, all in all, I'm inclined to believe that dragons are able to use their lair actions and regional effects once they're 101 years old.