Word meaning since its existence
Solution 1:
Existence is a period of time. In your example six-months.
Inception is the beginning of existence.
“Since” references a point in time, such as the inception.
Therefore, your sentence should be modified to either change “since” to “during” or “in”, or to remove or move “six month”.
The figure grew steadily during its six-month existence.
The figure grew steadily since its inception. [six-months ago.]
Solution 2:
The structure seems awkward. Maybe try switching it around?
From a promising start, the figure grew steadily for the first six months.