When I think back now, there were a few things that could have been early signs of being aromantic - if I would have noticed them.
I never really had a crush on anyone or been in love. The "crushes" I've had were just intensely liking somebody but not in a romantic way. I still like the idea of a relationship, things like cuddling or doing fun things together or even things that are normally considered romantic, or simply spending really much time with someone I really like. But all of that not romantically, just platonically.
When watching shows or reading fanfictions or books I always tried searching for the non-romantic ones and found it really annoying if the story was centered around romance, since I really didn't care about it. It was always kinda sad when I found a really good story, but then it got more and more romantic.
I've been in relationships before, when I didn't realize yet that I was aromantic. There were never really any feelings while doing romantic couple things and I remember wondering if I'm supposed to feel something while kissing, and things like that. Another thing that I never liked was being with a partner in public, the whole holding hands or even kissing in public thing. I always wondered if we really needed to do things like that and felt really uncomfortable and even repulsed by it. But everyone was doing it, it was normal. And apparently it was very important since the other person would get sad if I said that I didn't like those kind of things. It just was lost on me how those things could be so important so someone.
And also, I tend to feel awkward or weirded out if people show their (romantic) affection for their partner in public.
And another thing that just came to my mind, when I imagine myself in a few years, it's always either living alone or, if I have a family, it isn't with a partner or anything like that.