Learn English – “The pain is less now than it was 5 hours ago” or should it be “lesser”

Young is the time of life before adulthood, synonymous with youth.

When I was young. = When I was a youth. Before my adulthood. This wouldn't make much sense for a 10 year old to say, but they may. Also, someone that is 100 may use young to refer to their twenties, even though that is adulthood.

Younger is comparative with the current age.

When I was younger. = Ages prior to my current age. This obviously depends on context. If you are 100 then younger could be 90, however in the context of playing football that is not as likely.

You might want to look into these words:

1. stingy
2. cheap
3. miserly
4. tightfisted

Depending on some other context, you might also want to look at

1. frugal
2. scrimp

I forgot to include skimp! That one was the first one on my mind.
As StoneyB writes, we would say

Don't skimp on your child's education.

I gave a list because I was thinking of other ways to say this. Like "Don't be stingy when it comes to your kids education."