7 years old domain

i recently registered a "new" domain, at least i thought it was new. Now i checked it with domain age checker and found out its a 7 year old domain. Does it improve ranking and how can i check if it has any negative backlinks etc?

Ahrefs.com will tell you the backlinks.

(1) Check from archive screenshot if it has been used as PBN or some Chinese/Japanese lang content in the past
(2) Check link from majestic or ahref and look into the anchor

These 2 should give you enough to decide if any negative are thr

alright thanks, but does google see it as new or aged domain now after re registering?

Use ahrefs for getting backlinks list.
Use Wayback machine and majestic to check spam score.

That domain has no age in regards to SEO. Age of content is all that matters, not when was the domain first registered. That does not impact anything. You will receive a boost if the domain has some backlink profile in place, otherwise not. Also, make sure that the domain was not abused in the past, otherwise it may not rank, even if it's not manually penalised. Check archive.org and domaintools history. If it was used for affiliate purposes and obviously SEOed previously, then avoid it.

Also, do yourself a favour and disregard any automated "spam checkers".