It totally depends on how you feed them. If you power feed them you can get boas to breeding size before or by 2 years of age. They still won't be full sized for another several years though (if they live that long). Boas can reproduce before they reach their full adult size.
With the way I'm feeding mine, I expect my females to reach breeding size around 5-6 years old if not older. My males probably around 3 years, maybe 2.5 at the earliest. Just depends. Right now I only have one baby male but I do plan on adding more at some point.
At 5-6 I'm expecting maybe 6' of length, and it could take another 5-6 years for them to reach those big 7'-8' sizes (if they're meant to get that big). They might hit 7' before then but 8' would be a stretch if they ever reached it.
The same could be said for other species too though. I see a lot of retics 11'+ at 2 years but mine will be 3 in October and she's maybe 8' or a little over. I know another retic keeper who doesn't see full retic sizes until 5-6+ years. One of her retics was nearly 15' when she died (cancer), and she was conservatively fed very similarly to mine, and maintenance fed by her previous owners twice as long as mine was. River was maintenance fed for the first 6 months of her life (I'm talking a fuzzy mouse every 2 weeks when she should have been eating weaned rats), their's was maintenance fed for a year. Both were roughly 3' when we received them iirc. So yeah feeding has a lot to do with how fast a snake reaches adult sizes, within the confines of that species/localities' size potential.
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