30 was actually a tougher b-day for me than 40. Youth is once and for all officially R.I.P. at 30, most people are still in the phase of establishing themselves, income not where you want it yet, etc. 40 you already know you're old, are probably established, income probably increased, etc. So odds are good your life situation are better at 40 than 30 and you're not crossing some huge divide between "young" and "getting old" as you do at 30.