I'm a firm believer in love no matter the time or no matter the age. A 15-year-old can be in love just as much as someone who's been married for 15 years. Love knows no time, distance, or age (to a certain extent).
The quality of a relationship shouldn't be defined by the amount of time that two have been together. I know couples who have been together 5+ years and they are in a worse relationship than people who have been together for 5 months. Understandably, couples who have been together longer have passed the honeymoon stage and have been put through tough circumstances together.
They've had the opportunity to grow together and yes, that's something that a new relationship doesn't have yet. However, the love that you have in a relationship, no matter how long, should be what defines a relationship.
I've been with my boyfriend for 8 months now and the growth that we've had to go through due to certain circumstances makes us head and shoulders above some couples who have been together for years. We've had the ability to see what makes each other tick, what hurts us, and what makes us feel loved. That's something that some long-term couples might not have.
My point? Relationships should be worth more than a number of days. A relationship's worth should be determined by love, by trust, and by a genuine connection. If you don't have that in a long-term relationship but have it in a new relationship then sorry, not sorry, I will envy the new relationship every time.