I've seen some debates about Starlight's Age already but the last one was about 5 years ago so I thought about reopening that barrel.
My guess is that Starlight is about 5 or 6 years older than the Mane 6 since she grew up with Sunburst and he's definitely older than Twilight. There's might be a 1 to 2 year difference Starlight and Sunburst but that doesn't change the main fact for me that she's possibly older than Twilight or the Mane 6 in general.
I also saw someone saying that if she was older that it would be weird that she's so dependent on Twilight.
I think that's mostly because Starlight is scared, people know her as a villain and she's trying hard to be a friendly pony to everyone and she needs Twilight's constant reassurance for that since Twilight is somewhat her role model and also the Princess of Friendship. Some past trauma might be involved with her getting left behind from Sunburst and so she's desperately trying to become independent but she's failing cause she needs that reassurance she never got back then.
That's just my guess, feel free to debate with me ;3