It's easier to get in and out of a vehicle with seats about two feet off the ground, said Keith Knudsen, Ford vehicle architecture manager.

That's the emerging sweet spot as a wave of SUVs replaces traditional sedans as America's best-selling vehicles.

And so the automaker shift from sedans to small SUV-type vehicles could pay off in a big way as drivers grow older and learn to appreciate seats that are easier to slip into than low-slung cars or traditional taller SUVs and pickups.

A few examples include the Nissan Kicks, Chevrolet Trax, Ford Escape, Honda HR-V and Kia Soul. The vehicles also provide better visibility, thanks to their height and big windows.

Easy entrance and exit will become more and more important as the population ages and huge groups like Gen X move into their 50s and 60s.

"Seat height is key," Knudsen said. "People like to be able to slide in, not lift themselves up or down."

Vehicles designed to prevent injury

Automakers don't talk about this much. It's a truism that people want cars that make them look younger, not reminders they're less limber at 60 than 40. Nonetheless, the incoming tide of small SUVs will fit aging drivers and people with limited mobility like a glove.

"Front-seat access is the No. 1 factor in comfort and safe driving," said Sherry Kolodziejczak, national coordinator for driving safety at the AARP, which represents 38 million people over 50. "The small SUVs tend to have wider door openings and lower sills to step over. Step-in height is really, really important," not just for the elderly, but for many people recovering from injury or surgery, said Kolodziejczak, a professional occupational therapist.

"The vehicles are just easy to use. It's easy to see, easy to get in and out," Nissan human factors engineer Larry Smythe said. "People appreciate that."

The ideal seat height is about 21 to 27 inches above the ground, Knudsen said. Other factors include the size of the door opening, from top to bottom and front to rear. Nobody wants to bump their head getting in and out of their vehicle.

"Getting into a sports car is a controlled fall into the seat and a climb out," Smythe said. "A pickup is the reverse."

Like getting out of an office chair

By contrast, the small SUVs are designed so your hip is slightly higher when you stand outside than when you're in the seat. "You feel like you're walking in because you are," Smythe said.

The cushions on SUV seats are likely to be relatively flat, so you slide in and out, compared to cars which often have high bolsters for sporty looks.

SUVs' taller roofs allow the seat to be higher off the floor than most sedans. That allows for a more natural sitting position than in a car, where the seat cushion may only be five or six inches off the floor and your legs stretch forward rather than being at the base of the seat. It's easier to step out of the higher seat in SUV, just like it's easier to get out of an office chair than a beach chair.

"The seat backs tend to be more vertical than in cars, which provides a better path for your back to deal with jouncing from the road, and a more even distribution of weight over your thighs and butt," Smythe said.

SUVs' high roofs, greater ride height and bigger windows also contribute to better visibility, Kolodziejczak said.

Features like those ensure the SUV boom will be around for a while.