"Youth" has two sides. Personally, I usually find a girl at 25 more beautiful than she was at 18 – personality and confidence add something that no amount of biological freshness can beat. But that's a matter of taste, so I'll leave it aside.
The thing is, girls at the beginning of their careers almost always ask for less money. As in any profession (from medicine to IT), experience adds value.
Would you ever go to a well-established lawyer and say, "Hey, buddy, since you were getting 500 as an intern, now that you have an office in Kolonaki and are charging 5,000, you're a thief"? Of course not. So why should it be any different here?
Youth offers a spontaneity, a freshness! Most girls start out in adolescence, having an ignorance of danger. They have not been burned by the conventions of adult life. They have a well-meaning madness…
Unfortunately, however, some people are not (only) looking for freshness when looking for new entries. They are looking for something more vulnerable, more docile or easier to handle… They are looking for someone who, due to ignorance, financial need or lack of experience, will not be able to react, set boundaries or claim the respect she deserves.
And as soon as this girl matures, learns the job and stops being an "easy target", the cracking begins...
PS: Obviously I'm not being sarcastic, the comment is targeted.