I agree with Foxy, you are both right.
Genetic make up plays a major role in determining the hair's terminal length. Personal hair care plays a role in realizing the terminal length genetics has predetermined.
Terminal length is the longest length your hair will grow in its growth phase. But, terminal length is more of a time measure, than an actual length measure. Growth phases last years, and hair will grow until it hits that time point--- not necessarily a certain length point or marker. Hairs grow independently of the ones around them so when one reaches terminal length, another may just be a year or two in. If your growth phase is genetically engineered for 4 years whether you've gotten to waist length or been stuck at shoulderlength in those 4 years your hair will shed at that 4 year mark. Even if you shave your head from waistlength to bald at 3 years and 364 days in, at that 4 year point it will shed- regardless of finishing length. Your job is to simply bank as much growth and retain as much length as you can before the clock runs out for each hair. I don't believe you can truly know your terminal length until you've been growing your hair for several years on an impeccable healthy hair regimen.
Genetic make up also plays a role in how we age--- whether we'll start to thin out an an early age, experience a texture change, etc. As we age the hair's growth rate does tend to decrease. Hair grows the fastest in childhood, so maybe that is what she meant by you being 21 hitting your old age.
As long as you are alive though, your hair/nails will continue to grow no matter how old you are. Now how much of it you have left thickness wise is up for discussion.