This is the real beauty of universal design - the fact that a space can be enhanced for general safety, comfort, convenience, safety, enjoyment, or access by looking like it had been created this way right from the very beginning. When multiple people in a space - living there or visiting - can use something effectively even though they might be different ages, sizes, and possess various abilities, universal design has been achieved.
Then, there is the concept of adaptable or adaptive design where a space is made functional for someone who develops a particular need (or more than one) all at once in the case of a traumatic event or over time in the case of the normal aging process or a progressive condition.
If someone didn't know any differently, the end result of a universal design treatment done proactively when there was no specific need and an adaptable design done to facilitate a particular limitation, would look essentially the same, if not identical.
Take someone with balance issues. A hallway or public area of a home - and even bedrooms - could be outfitted with decorative chair rail and wainscoting that would allow someone to use the top cap or rail for support. If the need wasn't present, or not there all the time, the result would be a decorative finish to the home. Contrast this with railings hung along the hallway that call attention to someone's need to use them for support. They don't add anything to the design and actually detract from it.
Universally, the treatment is done now - before there is any apparent need - and in an attractive, unobtrusive way that looks like extensive trim and molding work. As an adaptive design, done in an attractive way rather than just installing some railings or bars along the wall, the look is the same, but the installation had to wait until someone needed it.
This is what I mean by timing and intent. Do it now, or early on in the home ownership period, because it looks good and can serve any number of people with varying abilities and needs, or wait until it is specifically needed. The outcome is very similar if not identical, but with the second one there is more of an urgency to get it done.
The same can be said for roll-under countertop space in the kitchen, bath, or laundry room that is left open all the time or has retractable doors that can open the space when needed and close it back up when not in use. It can have just the countertop bridging the space between adjacent cabinets, or there can be an apron or drawer that frames the opening under the countertop.
Light switches, solar activated lights, doorway that allow easy passage, floors that provide smooth and even transition from room-to-room, adequate lighting in a space with no harsh shadows being cast, and many other items of a safety, convenience, and use perspective can be included at anytime in a home's design and life - or they can wait until the need calls for their installation - often with less time for planning and decision-making.