I have fry after 3 months…

[plankton]

I'd agree with @black ghost, could even be slightly older, and probably female as the tails don't seem to have much (bright) colour yet.

[Hiya]

Thank you…!
Now this is probably a really dumb question but as I don’t know the answer I will ask anyway…? Can fish safely mate with their offspring without any detrimental effects? Obviously, I realise, there’s no way of stopping it.

[plankton]

I haven't added new stock for a long time in my guppy tank....and it's been running for 25 years.....does that answer the question?

[Hiya]

I haven't added new stock for a long time in my guppy tank....and it's been running for 25 years.....does that answer the question?

My first born youngsters are now occasionally swimming with the others in the lower parts of the tank. Slightly over an inch now. How old do you reckon?

[black ghost]

https://petfishonline.com/guppy-fry-growth-stages/

[plankton]

That's not a bad guide.
I'd say over 4 months if they're male they're just about fully grown at an inch or so (without tail length). Females are a bit bigger.

[Hiya]

That's not a bad guide.
I'd say over 4 months if they're male they're just about fully grown at an inch or so (without tail length). Females are a bit bigger.

I was including tail length in that. More like 3/4” without tail.

[plankton]

About 4 months then.