Welcome Dave!
I’m also a Dave (NH, USA) a few months from 70. I’ve been using the Justin courses for a while now. Here’s my experience relative to your questions.
Fingers… Yup, there’s a little pain hurdle to overcome. Play regularly, don’t over do it to start and the fingers will catch up and allow you to comfortably play longer.
Chord changes… Be patient, practice a set of changes at a pace that you can do. After a while you’ll start to feel more comfortable and the speed will increase. Rinse and repeat for other chord changes.
The APP… The app follows the course levels. I.e. there are songs that use only the chords that you have learned so far. You can slow down the songs in the app to match what you can do (e.g. play at 60% speed). As you progress, you can speed it up. I found it useful to work on chord changes and strum patterns as I improved. OMCs are a good practice technique but playing along with songs is more fun. The combination was good. I do no know about using it on more than one device. Musopia people should be able to address that.
Thanks, the pain is getting less noticeable !! 
Hi Dave, thanks for the reply and for the advice which is much appreciated 
I will get the APP and give it a try, hopefully it will aid my progress
Welcome Dave.
What fine travel companions you’ve given yourself as you approach a new decade. We’re here to walk your way, celebrating and enjoying your progress. Do let us hear how you’re going, won’t you?
Thank you Batwoman, yes I’ll keep in touch this seems a nice friendly place to be 
Hi Dave! Good to see you here!
I’m brand new to the guitar too, about three months in with Justin’s course. Finished grade 1 and staying put to solidify chord changes and get comfortable with the basic strumming.
I have the app, and I love it. The lesson videos on it for the courses are the same ones that are on the website, and on the app they currently only go through grade 2. BUT the play along songs on the app are awesome. And like David said, the tempo can be slowed down which is great while learning to speed up chord changes. There are hundreds of great songs on there to play along with.
As far as working on the speed of chord changes…doing the One Minute Changes (OMC) with a metronome has been a game changer for me! I saw it suggested by another member in this community…here is that post, it’s described there…
Happy playing, and enjoy yourself! 
I got the app, decided I didn’t really need it - and then immediately regretted that decision and got it again. I really like using it as an accompaniment for playing the practice songs. I’m not sure about Android devices but, I can confirm that I use the app on both my iPhone and iPad.
Hello Dave From Essex !!
Great time to start playing a guitar. My mantra ? “You’re Never Too Old To Rock and Roll” courtesy of Jethro Tull. Yep those fingers are gonna scream for a bit but hang in there and stick with the program and follow the path. Won’t be long before those dodgy digits are a distant memory and your belting out some songs.
Any question shout up here. No such thing as stupid questions, only stupid answers !
Cheers
Toby
Welcome to the community Dave!
Hello Dave and welcome. 
Like others have said, the finger pain does start to ease. The chords changes will come, it’s just about training the brains muscle memory but stick with it and you will get there.
Howdy Dave! Never too many Dave’s, right? Welcome to a great community. You can use the app on multiple devices as long as they are in the same ecosystem — i.e., all IOS or all Android. The IOS product and the Android product require separate subscriptions.
Thanks Dave, and everyone else for the encouragement and advice given.
Hi Dave,
First of all a warm welcome to the community 
. I have been exactly where you are 3 and a half months ago. My fingers would hurt a lot and I couldn’t play more than maybe 10-15 mins in a single session. But after a few weeks that started to go away. So hang in there, it does get better and soon you’ll be able to practice as long as you want.
About the app, it is really useful. It has practice sessions, logs and the best is the songs. It shows when to change the chords and gives hints about strumming. You can slow the tempo in the beginning and speed it up when you’re ready. I would say playing along the songs is what improved my chord changes. You’ll soon get to the video when Justin talks about that. I’d say this, after finishing up a few more modules, you can search for a song in the website, watch Justin’s video and play along in the app. It’s a gamechanger.
All the best Dave. The community has helped me a lot to improve and I am looking forward to your Journey 
.
Hello David, a very warm welcome and another opportunity to say how refreshing it is to see another mature player become a beginner musician. Awe inspiring.
When I saw your topic title, I couldn’t help myself, I immediately thought of the opening line of this song: Billericay Dickie - YouTube
@Richard_close2u now there’s a song that perhaps pushes the boundaries on content suitability 
And as it happens my own “Introducing yourself” title was inspired by the opening of I Wanna Be Straight 
Banana Rama by any chance?
I assume you are not asking me that question, Toby 
Just in case, to be clear, definitely not!
I was also tempted by this but although Dave is a match (and Dickie clearly is not) Dagenham is no longer in Essex. Dagenham Dave - YouTube
Dave - welcome welcome welcome! Never too late to start. It’s a wonderful community here. I hope you enjoy your journey.
P.S. the finger pain ends soon!
Cheers,
Vinnie
I went to school and grew up in Dagenham, it’ll always be in Essex on my mind😊