If a planet is of a certain degree, will it fully activate on that age? Suppose Venus is 18 degrees in the chart, so will it activate completely at 18?

It depends on your ascendant

It depends on your ascendant/Lagana degree.

Take this chart as an example:

You can see it’s an Aries lagana chart. The degree of lagana is 18 degree.

Lagana degree means when somebody was born, at that time which zodiac sign was rising from east at which degree.Here, aries sign was rising from east at 18 degree.

Venus , here is at 29 degree and it’s approx 10 degrees ahead of lagana degree. The more it is closer to your Ascendant Degree, more it will have an impact upon a person.

Planets always fully activate in their Mahadasha.

But to take advantage of it fully, you need to understand it’s placement and relationship with other houses and planets.