Depending on how you weigh bits of evidence, Danny, Swampy, and Bobbi could easily be anywhere from their early 50s to early 70s - but they're probably supposed to be about as old as Linda and Lawrence.
Danny, the lead guitarist of Love Händel, sings a song about the History of Rock music. The very first verse of this song is:
When I was a boy, down in South Illinois,
I heard a man playing blues, oh, what a wonderful noise
He had an old guitar, but not a dollar to his name,
Making music so sad, but he was happy just the same
He gave me a wink, and said: "Son, let me share the news...
If you want a happy life, you gotta learn to sing the blues!"
Danny evidently grew up in a time period when blues guitarists on the street were much more common; from the context of the song, Danny probably grew up when Blues was pretty popular. Illinois (and particularly Chicago) was one of the great areas of the United States for Electric Blues, and Danny performs this part of the song while playing an electric blues guitar. This puts Danny as having been a boy in the postwar era. Chicago's big electric blues era was around 1950 or so.
Later in the same song (which dedicates a verse to a bunch of musical genres), he continues with his upbringing:
I asked my daddy for a guitar, oh, I begged and I plead,
I said I wanted to play the blues, and he just nodded his head.
Daddy said when he was my age, "Boogie-Woogie" was the thing-
[Phineas] Just take the blues, throw out your hip, and add a little swing...
[Danny]: Ah, music has the power, that without it, he'd a-sworn
That he'd a never met my mom and I'd a-never been born...
Here, Danny is likely a teenager, since he wants to learn guitar. Boogie Woogie is pretty much the style directly predating the blues, and it makes sense that Danny's father would have been a teenager when "Boogie Woogie was the thing" (1930s-1940s). This is consistent with Danny being a boy in the '50s.
The next verse is about psychedelia, with no real lyrics and largely serves as an opportulity for the creators to throw in a disguised '60s drug culture reference.
The verse after is about Funk. It's pretty clearly supposed to be the '60s/'70s, but we get no indication of Danny's age in these years.
The final verse is about Metal, and is likely about '80s hair metal. Again, no evidence of Danny in the '80s (although he was probably a rock musician in some capacity), but this does reinforce the existence of the American musical genres. Also, Linda Flynn-Fletcher had a tour as Lindana in the year 1985 (in the credits scene of Phineas and Ferb Christmas Vacation, she receives one of her old concert T-shirts as a present, clearly labelled '85), making it likely that Love Händel formed after the Metal era.
Watching the whole episode, Lawrence says that he had bought tickets to Love Händel's farewell concert in the past. A few seconds later, he laments, "Well, Love Händel broke up years ago. Some things can never be recaptured." Linda and Lawrence have been married for less than the age of either of their sons, since in "Phineas and Ferb Get Busted", Candace has a flashback to how the family came together, clearly showing Phineas and Ferb to both be born before Lawrence appears in the Flynn household. I'd say that puts Love Händel as having broken up around 10 years ago.
So does it make any sense for Danny to have been born in the middle of the century? That would make him close to 60 by 2008 or so (more on why 2008 is a good guess as to the current year later). That's certainly possible, but the actual bandmembers certainly don't look middle-aged during their farewell tour at all. Depending on how literally you take the song, Danny could easily be closer to 70. I think it's likely that all the bandmembers are supposed to be around the age of Linda and Lawrence, because that makes for a more believable story, but that song Danny sings doesn't seem to agree.
If I had to guess, the song's primary purpose is to detail the history of rock music in a clever way - the fact that Danny sings in first person may be simply for musical or stylistic reasons, or perhaps he's just singing a song that isn't about himself at all.
Other points: One running gag that Love Händel plays out a couple of times is that whenever something ridiculous occurs to the band, they remark on how this reminds them of some leg of one of their concerts. As an example, in Phineas and Ferb: Across the 2nd Dimension, Love Händel's tour bus is tipped over during the robot invasion sequence, to which Danny gripes, "Oh, great. Albuquerque '93 all over again". This proves that the band was touring in 1993 and their farewell concert had to occur after that year, and is also consistent with Love Händel having "broken up years ago". The same thing occurs in "Delivery of Destiny", where after Doofenshmirtz Evil Incorporated is flooded with juice, Bobbi exclaims, "Hey guys, we're covered in juice and packing tape. What does that remind you of?", and Sherman responds, "Detroit 1984!", and he and Danny high-five.
In the episode "A Hard Day's Knight", Perry the Platypus is tasked with impersonating Dr. Lloyd Wexler at the annual Evil-Con in London. In order to be admitted to the event, he hands over Wexler's ID card for inspection, which reads:
Dr. Lloyd Wexler
NOTE: NOT A PLATYPUS
Evidently, Wexler's card is set to expire in April of the year 2009, and therefore the events of this summer seem to have to occur in the year 2008 or earlier. Episode 1 of the show itself premiered the late summer of 2007 in real life (and "Attack of the 50-Foot Sister" takes place during the "Midsummer's Festival", which has no specific date but undoubtedly would be held before August), providing some evidence towards the show being centered in the summer of 2008.
Additionally, it may be relevant to note that Love Händel's three members are each modeled after an important writer or creator of the show. Danny is named after Dan Povenmire, co-creator of the show. Bobbi Fabulous is named after Bobby Gaylor, who wrote some episodes and songs for the show (and also voiced Buford Van Stomm). Sherman/Swampy's namesake is Jeff "Swampy" Marsh, co-creator of the show. Povenmire was born in '56, Gaylor in '71, and Marsh in '60.