Future Trunks is in his early 40's by now?

[Angelus]

From DBS. Right?

[Nejishiki]

Future Trunks was born in AGE 766. It's currently AGE 797 in the alternate future. He should be approximately 31 (or 33, counting time spent in the Room of Spirit and Time).

[Angelus]

But Trunks was 17 when he first came into the main timeline. 3 years later, he came back for the Androids Saga and the Cell Games. He'd be 20 then. And then in that DBS episode, he mentioned he had to go back 17 years in time. He'd be 37 now then?

[Nejishiki]

Future Trunks is 19 (physically aged 21) when the Cell Game concludes and 22 (physically aged 24) when he kills Cell. The Future Trunks arc takes place in AGE 797 in the alternate future and AGE 780 in the alternate present. His approximate age using his year of birth, AGE 766, would be 31 (physically aged 33). We don't have his exact birth date, so the approximate figure was used.

Don't quote me on this part, but there's probably nothing wrong with thinking his birthday hasn't passed and he's 30 (physically aged 32), but this is the most literal interpretation anyone can possibly craft.

[Angelus]

I just don't get it. DBS Episode 47 has this conversation between Future Trunks and Future Mai

Future Mai: First, we'll head back to West City, 17 years back.
Future Trunks: Yeah. The world I went to, a long time ago.
[Flashbacks of SSJ2 Gohan beating up Semi-Perfect Cell, and then everyone saying goodbye as Future Trunks is about to board his Time Machine back to the future]
Future Trunks: That's where my friends are!

So the 17 years ago that Future Trunks is referring to is the Cell Games. If he was 20 or 21, when that happened, then he'd be 37 or 38 in the Future timeline right?

[Nejishiki]

He was returning 17 years from his current position. The current position was AGE 797, so 17 years into the alternate past would be AGE 780. I'm not sure how you're getting the idea he's trying to travel back to the Cell Game era. He never altered his time machine settings after his previous trip, before Goku Black, so time runs concurrently between both worlds. I mean, yes, that's his last memory of the alternate past, but you don't expect him to be aware of how the characters appear in AGE 780, do you? He isn't knowledgeable that Gohan became a scholar or that Goku and Vegeta surpassed Super Saiyan 2. I think you're taking this too literally. Because it takes a rigid imagination to assume Future Trunks referred to the Cell Game period, yet traveled later in the present future anyway. So, I'm sorry, but that's not right. I cited the dates for you.

[Gig]

It's not clear how much time passed between FT killing Frieza and him returning to the past to help fighting Androids. In the main timeline, three years passed... but it doesn't seem three years passed also in the future. Some time passed (Trunks required time -probably months- to get enough time machine fuel)... but IMHO he didn't look grown so much as if three years passed. However, we know that "aging" representation is somehow weird in Dragon Ball.... (if Future Trunks is gone 17 years back, then Kid Trunks should be around 17 years old? But he looks like 10-12!)
The series seems to imply that one has to use the time machine without making any change to the settings (keep past and future flowing "paired") to avoid creating further timelines, but IIRC at least in the anime, this has not been expressly stated. And the series does not pay excessive attention to "nerdy" details... they did not even give a bit of explaination about how Bulma was able to set Cell's time machine to reach Trunks' future. We can guess Future Bulma's notes allowed her to somehow "identify" the timeline, but there is nothing that states that.

In short, this is a mess. I think I'll just consider that Future Trunks is in 30's and Kid Trunks is a teenager... quite generic, I know...

[Angelus]

Yeah that's what I was basing it off too. Kid Trunks was like 2 years old when the Cell Games happened. In the Buu Saga, he was still under 15, which is why he's in the Junior Division of the 25th WMAT. I'm not sure how many years has passed since the death of Kid Buu up the Black Arc in DBS... but at least 13 years has passed then (15 years if you include Yo! Son Goku and Friends Return! as canon to DBS), since the last time Future Trunks came into the past?