Theme: Stuck in the middle with you (Stealers Wheel) - A partner to the target word (AGE) may be found in the middle of the theme answers.
17. Beverage from a German vineyard: RHINE WINE. New Age.
27. Unauthorized recording: BOOTLEG ALBUM. Legal Age.
44. Nuclear power: ATOMIC ENERGY. Ice Age.
61. Period after young adulthood ... and a hint to each set of circles: MIDDLE AGE
Argyle here. George Jasper appears to be a newcomer. Very easy except .... I think one might still find the AGEs without the circles. They aren't perfectly in the middle but they do span two words.
1. Stinging insect: WASP
5. Storied broom riders: HAGS. I didn't remember Hogwarts librarian Irma Pince.
9. Distinctive aroma: SCENT
14. Govt. workplace monitor: OSHA. (Occupational Safety and Health Administration)
15. Yemen neighbor: OMAN. They are the toe and heel of the Arabian Peninsula boot.
16. Copier need: TONER
19. Wheel spokes, geometrically: RADII
20. Triangular Greek letters: DELTAs
21. Coexist peacefully: GET ALONG
23. French fashion monogram: YSL. (Yves Saint Laurent)
24. Boot front: TOE
26. Anteater's slurp in the comic "B.C.": ZOT
33. Do the play-by-play, say: ANNOUNCE
36. Like Cheerios: OATY
37. Slightest: LEAST
38. Medical ins. plan: HMO. (health maintenance organization)
40. Tree trunks: BOLES
41. "To know me __ love me": IS TO. No need for sugar in your coffee this morning. 60A. Whitney Houston's "__ Always Love You": I WILL
42. Musician at ballparks and churches: ORGANIST
48. Put the kibosh on: NIX
49. Klutz: OAF
50. Noah's boat: ARK
53. One of a group of versifiers that included Wordsworth: LAKE POET. The Lake Poets were a group of English poets who all lived in the Lake District of England. Wiki link
58. Stubby piece: NUBBIN. A variant of Middle Low German knubbe, knob?
63. Excessive enthusiasm: MANIA
64. Partner of proper: PRIM
65. On a yawl, say: ASEA
66. Small hippo type: PYGMY
67. Trillion: Pref.: TERA
68. Mix together: MELD
1. Verbose: WORDY
2. Barbecue residue: ASHES
3. Con man's setup man: SHILL
4. Breathe heavily: PANT
5. Question in response to "I need this in a hurry": "HOW SOON?"
6. French friend: AMI
7. Criminal group: GANG
8. React to pepper: SNEEZE
9. Layer upon layer: STRATA
10. Mined fuel: COAL
11. Prefix meaning "within": ENDO
12. German no: NEIN
13. Pre-calc math course: TRIG
18. Go to a steakhouse, say: EAT OUT
22. Get dolled (up): TOG
25. Make an engraving: ETCH
27. __ buddy: BOSOM
28. Citrus fruit: LEMON. CSO.
29. Senseless: LOONY
30. South Pacific resort island: BALI
31. Sport-__: versatile vehicles: UTEs. Sport-youths?
32. Classic PC game: MYST
33. Et __: Latin for "and others": ALIA
34. Source of many tweets: NEST
35. Partnership for Peace international gp.: NATO. (North Atlantic Treaty Organization)
39. Sandwich cookie: OREO
40. "Baa Baa Black Sheep" wool unit: BAGFUL
43. Mom's mom: GRANDMA
45. Not out-of-bounds, as a ball: IN PLAY
46. AFL merger partner: CIO. (American Federation of Labor / Congress of Industrial Organizations)
47. Not subject to taxes: EXEMPT
50. Demean: ABASE
51. Bright Orion star: RIGEL. (Beta Orionis)
52. Work, as dough: KNEAD
53. Like a wet noodle: LIMP
54. On a trip: AWAY
55. Mattress option: KING
56. Kind of tournament round, briefly: ELIM. Usually, a single-elimination tournament.
57. Run out of gas: TIRE
59. Smile broadly: BEAM
62. S, SE or SSE: DIR. (direction)