How to group sizes with the Shopify 2.0 filtering systems?

Hi there,

I’m managing a baby apparel website (Shopify 2.0 / Theme Prestige 5.3) selling 900+ different products with many different sizes (listed on the left-hand column here as TAILLES: https://www.pepites.co/collections/tous-les-articles

).
Our distributed brands (30+) don’t all use the same size scales and some creators design evolving clothes that can be worn for several years.
A typical use-case I would like to tackle is related to a mother of a 4 month-old baby (of a standard height), who is looking to buy baby clothes sized 6 MONTHS. But she wouldn’t want to have to check off the options 6 MONTHS + 3-6 MONTHS + 6-9 MONTHS + 6-12 MONTHS + to hunt for any other listed option including the 6 MONTHS size.
She would like to see ALL the items in stock for her 4 month-old baby soon to wear 6 MONTHS clothes, whatever the size scales chosen by our brands.

Then I would like to sort the sizes in a logical order of age and no longer in alphanumeric mode
Today it’s listed as: 0 MONTHS > … > 3 YEARS > … > 6 MONTHS > …
I would like to have the sizes displayed as follows:
0 MONTHS > 1 MONTH > 3 MONTHS > 6 MONTHS > 9 MONTHS > 12 MONTHS > 18 MONTHS > 24 MONTHS / 2 YEARS > 36 MONTHS / 3 YEARS > 4 YEARS > 5 YEARS > 6 YEARS

Do you guys know if there is an app available to sort this out, any piece of code available for easy integration (my development skills are pretty limited) or should I hire a liquid developper for this job?

Many thanks in advance for your precious help,

Gilles