Inisfreeans do not mark time, nor do they age, in the same way or at the same rate as Outlanders. As Inisfreeans go through the physical developments of infancy and childhood, they remain inside their respective, individual cloning vat, aging just one Earth-year. When they are born from their vat, they have the form and full development of an adolescent human female -including all of an adolescent's desires- (never needing Outlander puberty), but the height of a toddler. While in their elementary-equivalent schooling in the Undercity (which includes the full battery of Inisfreean sexual education), their looks and form never change, except to grow proportionately until they are roughly 5-feet tall. From birth to Hatchling status, 10 actual Earth-years pass for them. Once graduated to the surface-realm of Inisfree, their expanding immediately slows down (a reversal of Outlander growth spurts), and their looks barely change at all for the rest of their lives. The more years pass for them as Hatchlings, Inisfreeans, and Master Females, the more slowly their height will increase. By Inisfreean-age 20 (the only point at which they will look slightly closer to age 19 than 15), they will have grown up to be between 5-feet and 5'9", they will never grow taller after this age, and they will still, as always, look like fully developed teenagers. Their engineering, educating, farming, dieting, exercising, and other lifestyle signatures keep them effectively immortal in the most beautiful, youthful form imaginable.
From Inisfreean-age 10 to 20, each successive Inisfreean-age takes much longer than the one before it, so although technically they do graduate from their city's sole High School within 16 actual Earth-years of being alive, their bodies will continue to look about 15-years-old long after this date. Below is a chart to better explain:
(BMI: Body Mass Index, WHR: Waist-to-Hip Ratio, HWB: Hip-Waist-Bust ratio)
So when an Inisfreean says they are 20 years old, that means they have reached their Inisfreean-measured 20th mark of forming and training (appearing physically to be about 19 to 20 at the oldest), and are hundreds of actual-years old, if not much, much older. Inisfreeans don't see this as lying or twisting the truth; they choose to say their age in this way to simplify things for those who are not yet ready, in their opinion, to learn of the details.