Response to comments under the other answer (which changes things substantively):
Each point in the X axis is age in months (i.e. 0 months means less than 30 days, 1 month means 30 days from birth etc) The Y point is kilometers covered by the vehicle. So ideally as the vehicle passes through 0,1,2,... n months the distance covered keep increasing until the vehicle gets calibrated.
If you have many observations on each vehicle then you should not be fitting an ordinary regression model -- the data for a single vehicle are dependent for two different reasons. Firstly, you're observing the same vehicle, which leads to dependence (repeated measures), but secondly you're looking at cumulative totals. This makes the values highly dependent.
You should be looking at how much was added each time. This removes the effect of the cumulation (and may also remove much of the fanning effect).
You still need to account for the dependence between observations on the same vehicle in some way, either via a model than accounts for them as repeated measures or via time series models.
The values you're observing will necessarily be positive (or at least non-negative); this will tend to lead to skewness, and may make models designed for a normal response unsuitable. You might need something like a generlaized linear (mixed) model, or you might need to consider a transformed model, for example.
That is why at 0 (i.e. intercept) it is also important for me to infer the kilometers covered by the vehicles.
If I understand (and I am still not sure what it is you're trying to find out here), I think the information you're after should be in each month's incremental data
I have tried to remove outliers by taking the 90th quantile for every age (i.e. 0,1,2,3,..,11)
If you're after averages this may not be suitable (and even if outliers really are something you need to remove rather than model, I doubt this is the best approach) ... but I'm confused are you going across vehicles when you do this? I am not at all sure that makes sense.
I have a feeling we're dealing with an XY problem here -- that instead of explaining the underlying problem you're trying to solve, you're focused on asking about your attempt at a solution. You may be better writing a new question clearly describing the underlying problem more explicitly.
[Original answer removed since in light of the new information, it's no longer relevant]