The astronomy behind the angle
An opposition is one hundred and eighty degrees of separation, half a circle. Uranus needs roughly eighty four years for a full lap, which puts the halfway point in the vicinity of forty two. The measurement is of ecliptic longitude only, so it is the same calculation whether or not you can see the planet.
Nothing about the aspect requires the planet to be doing anything unusual. Uranus keeps to its ordinary pace, and the opposition is a geometric fact about where it stands relative to a fixed degree in your chart.
An eighty four year period is convenient in one respect, since it means a single lifetime can contain the opening square, the opposition and the closing square without ever containing a return. Only the very long lived meet Uranus back at its natal degree.
The transit is also independent of the sign involved. What is measured is the arc between two longitudes, so an opposition from Scorpio to Taurus and one from Virgo to Pisces are the same aspect, described by the tradition in terms of the houses and the natal chart rather than the signs alone.
Why thirty eight for one person and forty four for another
Orbits are ellipses, not circles. A body moves faster along the part of the path nearest the Sun and slower along the far side, so the time needed to cover half a lap depends on which half you start from. Someone born with Uranus in the quick stretch reaches the opposition closer to thirty eight, and someone born with it in the slow stretch waits until about forty four.
That is why the age is quoted as a range. The average of forty two describes the population and not your chart, which is what the widget above is for.
The same reasoning explains why sibling charts a few years apart can produce oppositions five or six years apart in age. The order of birth tells you nothing useful about the order of the transits.
Three passes and a long middle
Uranus is retrograde for roughly five months of every year. Because it also moves slowly, an opposition that falls anywhere near a station is crossed three times: forward, back and forward again. From first to last that commonly runs from several months to a full year, and single crossings are the exception rather than the rule.
The tradition treats that whole span as the transit rather than picking one date from it. Every crossing is listed above, along with the gap between the earliest and the latest.
A three pass opposition is not a stronger transit than a one pass opposition, only a longer one. Astrologers who distinguish between the passes usually treat the first as the arrival, the middle as the reconsideration and the last as the settled position.
The midlife framing, and where it lands
Uranus is conventionally read as disruption, independence and a refusal of arrangements that have gone stale. Applied at forty, that reading became the astrological version of the midlife crisis, which is a much stronger claim than the tradition itself makes. Older texts describe restlessness and a wish to revise, not the abandoned marriage and the new motorbike. Difficulty is part of the description too, since the same restlessness is read as impatience with commitments that are perfectly sound. This page predicts nothing.
The house holding your natal Uranus is where any reading concentrates, and that needs a birth time. The dates themselves do not, because Uranus moves so little across a day.
Whatever else is happening in the chart at the same age matters too, since a Neptune square commonly falls in the same few years and the two are rarely disentangled in popular accounts.
Questions people ask about this
What age is the Uranus opposition?
Roughly thirty eight to forty four, most often around forty two. The variation is genuine and comes from the shape of the orbit rather than from any disagreement about method.
Is the Uranus opposition the midlife crisis?
That is the popular label rather than the technical one. Astrological convention reads the transit as a period of restlessness and revision, and the crisis framing is a much stronger claim that the tradition does not really support.
How many times does it happen?
Once in a lifetime, but usually across three crossings of the same degree because Uranus turns retrograde for about five months a year. The span from first to last is often close to a year.
How long does the Uranus opposition last?
Take the first crossing and the last as the outer limits, which commonly gives several months to a year. Astrologers usually extend the reading a little either side of that window.
Do I need my birth time?
Only for the house. Uranus shifts by a tiny amount in twenty four hours, so the dates hold without a time, while the house that gives the reading its specificity does not.
Does everyone experience it the same way?
No, and the tradition does not say they should. The house involved, the aspects Uranus makes in the natal chart and everything happening in an actual life all change what is read into it.