let's say something got fat in their 20s and 30s, but in the 60s can't get rid their fat because of slowed metabolism etc. Are they sinning?
There is no sin that would carry a process of repentance that takes 30 years.
What if all sins had a physical manifestation that was very difficult to change? Maybe the person changed, but the sin would still be showing? Therefore, they are sinful? Of course not.
Being fat is not a sin and not against the Word of Wisdom, even though it is not wise.